I am currently in love with Melissa Clark's cookbook In the Kitchen with a Good Appetite and she has two cakes that sound perfect for something like this. She has a dark chocolate bundt cake that is soaked in rye, it is definitely not sweet but very very special!! I made it for my brother in law's birthday, he also isn't a fan of sweets but it hit the spot! She also has a blood orange cake that is served with a blood orange compote on the side and it is delicious, simple, and you can adjust how much honey you put in the compote to make it less sweet if you like

And to continue the orange theme, I just made this cake this afternoon (http://traps.posterous.com/orange-chocolate-loaf from http://smittenkitchen.com/2006/09/this-cake-has-a-hole-in-it/)

..but without the chocolate ganache, as I thought it might be too sweet. Although if you used 70% dark chocolate it might not be as sweet? I also halved the ingredients and used a tad bit less of sugar, and once baked, spread a thin layer of orange marmalade on top.

Hope you find something that works well for the occasion!

I like the idea of a cheese board, maybe with some honey alongside and some savory herbed shortbread?

I also think strawberry shortcake would be a great idea- you can use a biscuit recipe that isn't very sweet (I like smitten kitchen's cream biscuits) and you barely have to sweeten the strawberries, just a touch of sugar and lemon juice will get them to macerate. Top it with unsweetened whipped cream and you have treat that will satisfy anyone's dessert craving without very much sweetness.

I'm also not big on dessert, and to complicate matters, I don't eat wheat. But I've found some things that I do like for occasions where cake is called for:

Crustless cheesecake: http://www.cookingdebauchery.com/cooking_debauchery/2007/02/the_cheesecake.html
I used honey in place of the sugar, and much less of it, so the cake was just slightly sweet. It's good topped with cooked down strawberries.

And flourless chocolate cake from America's Test Kitchen is also good. Basically just a pound of dark chocolate, 1/2lb butter, and 6(?) eggs. all mixed together.

What about a sushi cake for dinner? Lots of recipes online and can be as fancy as you'd like to make it. It still has a "cake" aspect (you could stick a candle in it), but it's a main course rather than a dessert. I'm not a dessert person and one year my step-mother made me a sushi cake and it was thoughtful, beautiful and delicious.

I'd turn to the raw food world on this one. Raw food desserts don't have any processed sugar and are quite tasty, in addition to being rich enough for a special occasion and healthy enough for your needs.

Angel food cake! I just made one yesterday for Easter at my boyfriend's parents' house. I just rediscovered how awesomely light they are, and really not too sweet.

there was recently a recipe on here for yogurt cake. we made it for my sister's birthday (she isn't a cake/sweets person at all) and she loved it! we topped it with fresh whipped cream (only slightly sweetened) and strawberries. it was a huge hit.

What about an olive oil cake without any frosting. You could even add rosemary.

Tiramisu! My husband is not a "sweets person" either, but he LOVES a good, homemade tiramisu. I use a family recipe, but this one is pretty close:

http://allrecipes.com/recipe/classic-tiramisu/detail.aspx?event8=1&prop24=SR_Title&e11=tiramisu&e8=Quick%20Search&event10=1&e7=Recipe

To simplify this recipe:
- use less sugar; 3/4 to 1 cup is enough (especially if your bf doesn't like sweets)
- no need to put eggs & sugar over double-boiler
- don't bother to split the lady fingers
- add 2 cups of cooled espresso/coffee to the liquor to quickly dip the ladyfingers instead of brush

Also, I just layer everything (dipped ladyfingers then cream and repeat) in a big glass baking dish. It's more rustic but just as pretty (and delicious) with a sprinkle of cocoa powder or chocolate curls.

Hope that helps! Enjoy :)

What about a polenta cake? Those aren't too sweet and you can cut out the glaze. Just add berries.

I love a clafouti too -- there's a good recipe with blackberries and thyme from Chef Love. I actually make it with less of the sugar and it doesn't hurt anything. http://www.cheftimlove.com/Press/InStyle051910.pdf

Or Mario Batali's pastry chef has the recipe for her saffron panna cotta online. It's amazing. They have a rhubarb compote with it too. Not too sweet either...http://www.babbonyc.com/dolci-rhubarb_panna_cotta.html

Try an olive oil or buttermilk cake to change it up and still keep the cake aspect while getting you well out of sticky-sweet territory. Also seconding strawberry shortcake, you can use pretty much an unsweetened or only lightly sweetened biscuit recipe--I just did this the other day, 2 cups flour sifted together with 1 tbsp baking powder and 2/3 tbsp salt, cut in a stick of butter with a pastry blender, put it in the fridge for like 10 minutes to harden up the butter, then add 1/2 cup milk, 3 tbsp sugar, and 3 tbsp melted butter, mix together (it will be dry so you might have to use your hands in a kneading motion rather than a spoon) and then chunk it up into 6 balls/lumps, put them on a lined baking sheet, sprinkle with sugar (or don't) and bake at 425 for 15 minutes.

You can also adjust the sweetness way down on a cheesecake and it will still be delicious, especially if you use a tangy soft cheese as part of your base and include supplemental flavors like citrus--maybe an orange or lime cheesecake? Cookie crumb crust recipes (graham cracker, chocolate, etc) also often can have a lot of the sugar removed without effecting the structure of the crust, which is mostly about the butter and crumbs.

You can tone down the sweetness without losing the freshness of a good fruit salad by including some chopped cucumbers, using fresh and not frozen fruit, reducing sugar in the dressing or subbing it out for a more complex flavor like balsamic vinegar, and using unsweetened (or less sweetened) whipped cream to top.

And speaking of tangy cheese and the fruit-balsamic combination, what about something like this: http://simplyrecipes.com/recipes/ricotta-mascarpone_mousse_with_balsamic_strawberries/ would definitely be pretty enough for a party.

Mango sticky rice!

Have you asked him what he wants? In my family the birthday person gets to choose their cake/sweet. I've known people pick ice cream or pudding. If he really doesn't want something sweet, why not fix a meatloaf cake?

I'm a fan of simple fruit tarts with a pastry cream and fresh fruit topping. Panna cotta can also be made to not be overly sweet.

SMORGASTORTA!
The Swedish Sandwich cake, oh so pretty, easy to make. Traditionally made with smoked fish and seafood, but you can switch it up to a blt version or turkey club.
http://1000vistas1000meals.com/2012/04/02/smorgastorta/

I agree with the other commenter who suggested angel food cake. That with an assortment of fruit, or balsamic-mascerated strawberries (with a dash of mint even?), could be awesome.

I don't know if you'll be in a place where you can pull this off, but this is my favorite not-too-sweet-but-total-showstopper dessert:
Assemble nice plates of fresh fruit, maybe a piece of chocolate or two. Then, take a hot skillet (off flame) and pour in Bacardi 151. Set it alight and pour over the dessert plates (with the lights either off or dim). Total showstopper, and works with just about anything.

How about making cornbread, popovers, savory muffins, biscuits, quiche etc...some sort of cake-like dish that you can place a candle into.

How about a spiced carrot cake? The cake itself is not that sweet and you can always control the amount of sugar in the frosting (or use less of it - just on the top of the cake). Another suggestion is a basic chocolate cake that uses olive oil instead of butter. Again, you can leave out the filling and just frost the top of the cake.

How about chocolate-dipped strawberries? It's mostly fruit -- not too sweet if you use dark chocolate -- and fancy enough to be festive.

How about poached fruit with ice cream? Or a fruit salad? We often have a fruit salad, or macerated berries with a little bit of yogurt and mascarpone lightened with whipped cream and barely sweetened if we're looking for a light, but not very sweet or rich dessert. I also think the cheese plate idea is a great one if he enjoys cheese!

Sorry for the atrocious run-on.

Cotton Soft Japanese Cheesecake. It's light. fluffy, and not too sweet.
http://www.thelittleteochew.com/2009/09/cotton-soft-japanese-cheesecake-for.html
Or
http://www.thelittleteochew.com/2011/03/steamed-egg-cake-ji-dan-gao.html