There's this one horse I ride who's always extremely heavy on the forehand. It affects his performance as well as my riding. On him, I always find myself leaning waaay too far forwards, my reins slipping in attempt not to yank on his mouth (even when I'm following his face with my elbows and hands), and my entire body falling forwards onto him over jumps. This NEVER happens on any other horse (and I've ridden plenty of horses recently) so I know it's not just bad riding habits. It's just whenever I get on him that this happens. From the way he moves I can tell he's extremely heavy on the forehand and his hindquarters are far from engaged. His head drops low, especially when he canters. His strides are really slow and lazy and stressed. None of the motion is powered from his hindquarters. It's not a pretty sight, since as a result both horse and rider end up looking messy. How can I engage his hindquarters, get him moving, and get him lighter on the forehand? Also, are there any riding errors I might be making that are contributing to the problem?
**NOTE: I know I don't hang onto the reins for balance or support, and I know I don't have a super tight rein. With this horse, if you ride with anything but a soft but steady hand he's all over the place, going everywhere and yanking down at the bit and messing up all his striding. He's taught me a lot about having a nice steady hand. I used to ride him a lot when he was green and we came a long way, then when he started being really good everyone loved him and lot's of people started riding him. Once I started riding him again, there were all sorts of new problems. He never had this problem before.