This is one thing I don't enjoy doing, but if you have a grassless paddock or a large stall (so he can keep his legs stretched), try keeping him in there with no pasture turn out and feed him a normal amount of feed. Up his exercise so he is ridden every day for at least 30 minutes. No treats!
If that doesn't work.... turn to your vet. He could have some kind of internal problem.
Good luck!