how much are u feeding him? they eat very little,just 2-3 pellets a day.feed one at a time.to make sure he eats it.has he pooped?if not u can try a cooked frozen pea.it has to be a frozen pea thawed out and shelled,then mash it up and give him 1/4 of it.this will help him poop.after this u can feed him 5 days fast on the 6 day feed pea on the 7 day.the pellets may be to big try crushing them,giving him betta flakes,blood worms,brine shrimp.i'm new to keeping bettas too.go to ultimate bettas.com good site.or betta talk.com.

I still can't get him to eat. I tried changing his water, giving him different food (betta flakes), but he still doesn't seem to be eating. I tried the pea, which he ate, though he spit a little bit of it back out. He does the same thing with the flakes. He only eats one or two, then spits a little bit of them back out.

Whats the water temp ?

It sounds like a water problem not food. Are you changing some of the water ?

Try changing some water (24 hour aged water or dechlorinated dont overdose on dechlorinator) .