What you are looking for is more hindquarter engagement - you will need lots of transitions to get the horse to "sit" more behind, to balance more of it's weight on the hindquarters than on the forehand. This is not a simple quick fix, it will take many months of progressively building your horse's ability to take more weight behind, and he will need to build up the muscles and fitness to be able to support this.
Try google for "how to improve hindquarter engagement"
Exercises to engage / quicken the hindlegs
1) Ride lots of transitions WITHIN the pace (in trot, canter) ride in working trot, then push it forward energetically for a few steps then transition back to working trot...repeat this several times on both reins and on circles, straight lines, across the diagonal - until the exercise becomes easier for the horse
2) Ride forward as though you are about to transition to walk, but then ride forward from it, without actually walking Why? this exercise helps to shorten the horse, to keep it more together, and makes the hind legs quicker by making the horses hind legs come more under his centre of gravity, which will progressively build the muscles needed for him to "sit" more behind.
Try this! ) Good luck, and more importantly have fun