My horse keeps losing weight. :(?

I have a 7 year old OTTB, he gets worked steadily and a few months ago i finally got weight on him, but now he is losing it again. He has had his teeth done, and is wormed regularly. He was coming in every night, but he circles, so he goes out almost every night now, lots of gr, hay 2x a day, and he was fed one scoop 2x a day plus weight builder. and he started losing weight! So now he is up to two scoops 2x a day, but why could he possibly be losing weight? It's good hay and the feed has a good fat percentage.