Sounds to me like an immaturity problem.
This guy got lucky and got a girl to indulge his fetish, but the experience has gone to his head and now that things have to change, he's thinking he doesn't want to not be able to enjoy a fat and gaining gf so he's going to cut off his nose to spite his face, mistakenly thinking he can just ditch you and find another girl who will continue to satisfy him.
Most likely he'll selfishly leave and break your heart without much concern, only to come crawling back in the following weeks/months saying how sorry he is and how wrong he was.
Or maybe I'm just being cynical. :/
Either way, I'm sorry that you have to go through this, I hope it works out for the best for you.
11 years