Well it's a good sign he's eating, but it sounds like the poor guy is sick.
My two bettas are greedy little pigs. I was told to only give them two or three of those tiny pellets twice a day or they would bloat up and get constipated.
That may have happened to your greedy little pig. You could try not feeding him for a few days and then giving him a partially cooked pea with the skin removed. It's supposed to be a great laxative and a cure for bloating.
I would go with the bettaMax and do 50% water changes once a week if you have a filter and twice a week if you don't. Best treatment for body fungus is antibiotics and lots of fresh, clean water. Is the betta alone? You should probably move any other tank inhabitants if you are going to use bettamax.
Hope your betta gets better!