Every betta has a different personality and preference in terms of food. As stated by others, he may still be adjusting, some bettas take a 3-5 months to fully adjust. Try a different brand of betta food, I would suggest either Omega One or Hikari.
Keep the temperature at a steady 28 degrees C all day and add a bit of aquarium salt. The combination will perk him up and speed up his metabolism.
This may sound cocky, but trust me on this, I know what I'm talking about lol. I've raised and bred over 300 bettas with the past 2 years and currently have a 25+ betta collection, comprised of all import show quality bettas, from Halfmoons, Halfmoon Plakats, Crowntails and even wild caught wild betta species.
I'm carious though, how often are you doing water changes and how much water do you usually take out?