That sounds like bloat and if it is you can help him if it isn't too far advanced with bettamax. Most all pet stores carry it. My betta got bloat and we were too late to save him. You will need to act quickly and get the meds fast. Make sure you speak to the fish specialist at your pet store and tell them his symptoms as maybe now there is a better med than bettamax. Once they get bloat it won't be long if it goes untreated before it will take their little lives. Good Luck