Lot's of hill work, going down is just as important as going up, and speed isn't important, walking whilst working from behind, is better than galloping flat out, plus doing transitions within the gaits i.e lenthening and shortening the stride. Rememer just because your horse is in an outline, it does not mean that  he i using his backend propely, when he is light on your hands and springing off the ground, he's working corectly.

Hi!

i would just like to share my opinion here... i dont think you should ned to use training aids.riding your horse in a long & low frame should increase his rear end and back strentgh alot. And alot of trotting, i mean miles of trotting. when schooling do alot of transitions from trot, walk, halt, rein back, trot, walk halt rein back. This all has to be done in a "round frame" and i dont neccisarily mean his head " on the bit" but his whle body rounded, his back up and carrying your weight, and his hind legs pushing him along, rather than his front dragging the rest of him. the hill work is fantastic aswell as riding/lunging in deep sand.

there are some chiropractic stretches you should try to supple him up before riding aswell... they are very simpleand if you interested in learning them message me =)

You don't say what his age is or how much he has been driven in the past.  Sometimes horses which have been driven are stronger up front than behind due to the muscles they have had to use to pull the cart.  Try lungeing him in a pessoa training aid to bring his legs under him properly so that he uses his back properly, this will help to develop his quarters.  When riding try to keep him off the forehand, some dressage work might help, you could enlist the help of an instructor for this if you're not too sure what exercises to do with him.  It will take time to get the back end working as it should but I think this is more an exercise and schooling issue rather than a feeding one.  Good luck.