What BRAND of food do you give him? Look at the ingredients on the label. As Aquatail mentioned, foods that contain a lot of grains or fillers can cause constipation, bloating, and buoyancy issues in some bettas.
As Hallyx mentioned, the two most often recommended foods on this forum are New Life Spectrum Betta and Omega One Betta Buffet. They contain quality proteins, and are well tolerated by bettas.
As Hallyx also mentioned, I would avoid anything freeze-dried. They pull moisture from the intestinal tract, which can lead to constipation (which in turn can lead to bloating and buoyancy issues). Frozen and live foods are good, though.
(Actually, the "ideal" food is live mosquito larvae. This is what they eat in their natural habitat. Depending on where you live (and what the season is), these may or may not be available right now.)
If this was my fish, I would try using Epsom salt at a dosage of 0.5 teaspoon per gallon. A lot of people recommend using 1 teaspoon per gallon, but I've had good results with the lower dosage.
Here is one way to add Epsom salt to your tank:
1) Put some tank water in a cup.
2) Stir in the desired amount of Epsom salt.
For example, if you want a dosage of 0.5 teaspoons/gallon:
--- 1 gal tank - stir in 0.5 teaspoons Epsom salt
--- 3 gal tank - stir in 1.5 teaspoons Epsom salt
3) Stir until the salt is COMPLETELY dissolved.
4) Slowly add the solution into the tank.
When pouring the solution into the tank - I don't add more than 0.25 tsp/gal every 15 minutes. This allows the fish to become acclimated to it. IE:
If you're adding 0.5 tsp/gal, you should pour in the solution over the course of 30 minutes.....
If you're adding 1 tsp/gal, it should take an hour (or longer) to pour it in.
Epsom salt can take 1-2 days to have a laxative and fluid reducing effect. Once they poop and are swimming normally, I do water changes to remove it from the tank.
Also, salts don't "evaporate." So once you've added it, you don't need to add more (unless you want to increase the dosage). And the only want to remove it from the tank is via water changes. (Unless you have live plants, which use it as fertilizer.)
(If this post isn't clear, just ask for clarification. It's 1:30am here, and I'm feeling a bit incoherent.