My Saddlebred, Cassidy, needs to get some muscle built up in him back and rump. The previous owners, had him along with several others that they couldn't afford to feed, and didn't take the time to work them all. So, he lost quite abit of weight and muscle. I'm wantng to get his back and rump built back up, but I'm not sure how to focus on those areas when we're riding. Any advice?