I know EXACTLY how he got ridden so much without developing his topline. He was a camp horse that got ridden by people who held their reins wrong and didn't have a good seat. That not only NOT develops the right muscles, it develops all the wrong ones. I have a friend that rides with her hands way up, and her horses are so ewenecked and hollowbacked that you can't believe it. She wants to do reining and can't figure out why she isn't winning and why all the trainers at the shows make such fun of her and her horse. You can't tell her anything, either. ; )

All you need to do to correct it is just ride him in the correct frame. It will take some time for his muscles to develop, just like with ours, but eventually he will be nice and muscled along the top. If you know how to lunge him with side reins or running reins (I prefer these to side reins), you can hasten the process, but be careful as you can make a horse sore if you overuse them.

Hills will probably help with his butt and back, but you say his butt doesn't need any work! Jumping (lunge him over them if you don't care to ride over jumps) will also help. The jumps don't have to be big (mine only go to about 2 feet, and I generally have them much lower). At least that's what my vet told me to do to strengthen a horse's back and butt.

Hope this helps a little!

Meezer