Some questions:
1) What's the water temperature?
If they're cold, their metabolism slows down. Bettas are tropical fish, and prefer water temps around 77-84F.
2) What brand of food do you feed him?
Foods that contains a lot of grains/fillers can be constipating to some bettas. Look at the label on the container and see if contains a lot of things like wheat, corn, soybean, gluten, etc. If it does, consider switching to a higher-protein pellet.
The two most frequently recommended foods on this forum are: New Life Spectrum Betta pellets and Omega One Betta Buffet. Both contains lots of proteins, that bettas can digest easily.
Next.... ich:
The easiest way to treat ich is to raise the water temp to at least 86F. The higher water temp prevents the parasite from reproducing.