What are your temps? They need heat to properly metabolize and digest their food. When they hold water or are well hydrated they appear fat. Also if your frogs eating regularly then the right side is probably full of poop and the left (being the stomach) food. If he is acting normally then he is probably fine. For feeding the best thing to do is a schedule. If he is a baby or juvinile he can be fed daily or every other day. Feed him as much as he will eat in 15 to 20 minutes so you keep track of how much he eats.

Pics would help. Different angles so we can see his side properly.