Help!? Something is wrong with my beta fish!?

Ive had him almost two years and he's always been a very healthy happy fish, my beta before him lived to five so I ume I take good care of them. About three months ago he started refusing to eat and I've tried everything to make him eat again! He's a very temperamental fish, and about the time he stopped eating I had bought him a new tank and my Dane puppy ate his food so I bought a different kind. I can't figure out which factor has caused him to stop eating! I put him back in his original tank with the same pebbles and decor. And I've bought almost every brand of fish food trying to find the one he likes! Most of the time he'll get a pellet in his mouth and spit it out. I thought they might of been to big so I break them in half and sometimes he still spits them up. I'm afraid he's dying because his torso is about three times the size of his lower half. Can anyone help?