Does your horse have loose/weak/sticky stifles? If your horse has weak stifles then getting your horse properly fit and conditioned is very important, he will need a program of regular, consistent exercise to build up his fitness and condition, concentrating on building up his hindquarters. Just remember to do it gradually, be careful you don't overdo it at first.
Ground poles progressing to raised ground poles and then to cavelletti as he gets fitter are all good for helping build up the hindquarters. Transitions are also great, try to add in lots of transitions during your ride, walk to halt, halt to walk, trot to walk, walk to trot, trot to walk to halt and so on. He should move forward into the bridle reaching well under himself with his hind legs when ridden to really use his hind end properly, he shouldn't be trailering his hind legs out behind himself. You can help him out by riding him forward from back to front, leg to hand, using half halts as necessary. Always remember to think of riding him forwards into the bridle from back to front. Hill work is also great for strengthening the hindquarters, if you have access to trails or fields with hills to ride on. Begin with walking up hills progressing to trotting up as he gains fitness and condition.