Well I had the same problem about 2 years ago with my horse...
1. We put her on a weight gaining supplement
2. We gave her free choice hay 24/7
3. We used a bungee system and walked/trotted for about a month working on turning and balancing.
4. Then we added a balanced canter to build her butt muscle..
It worked wonders..You just have to have patience it is always easier to take weight off then it is to put it on...