I agree with everyone here. I would suggest lunging him more then 3 times a week though, that'd be cutting him even on weight. How fat is this horse. Do you have any pictures. ? Unless he's close to foundering.. his weight shouldn't be such a huge problem. I have a super fat pony who jumps his height and no matter how in shape he is, he stays completely fat.. I wouldn't overreact so much over his weight unless it's a health factor. He can use 10 minutes of lunging a day. 5 minutes of trotting in each direction.. but keep in mind excersizing will lose his fat, but he's going to gain muscle too..