No, HE didn't let himself go, YOU did. He needs to get blood work done-he likely has EPSM and/or nsulin-resistance. He has gained a lot of weight in a short amount of time, at least a couple hundred pounds since October. Feed his hay in small portions 4-5 times a day, not free choice, and decrease the total quantity you give him. Exercise him daily, if you can't ride him get someone else to. Keep an eye out for signs of founder.