help! my betta isn't well

I've got a male betta, had him for about 8 months now in a 2 gallon tank, heated with water changes every 2 weeks or so. Lately all he's been doing is sitting near the top of the tank, barely moving at the very back. He used to come forward all the time and swim around alot. There's been no changes in his diet or environment and I don't see any signs of sickness.

I'm worried and don't know what to do :[