Riding up and down hills is awesome for building muscle. Transitions are great as well. Go up and down between walk/trot/canter, never going more than a few strides of each. This will really get your horse sitting back and using their hind end.
I found that lungeing with side reins helps alot. It helps them learn to keep their head down and also makes them work the hind end. The reins don't need to be tight, just enough to keep his head down. Just be careful not to over-do it. Like I wouldn't lunge him in side reins for an hour every day, lol. 10-15 min at first, maybe every other or every third day. It really helped my mare.