First of all I was wondering if you have asked your trainer how to correct the problem.Seem to me thats the place to start.If thats not an option try to change back and forth between a trot and lope.Transitions help to keep the pony light on the bit. Also you may want to do some backing and hill work to build up his back mucles to help him collect better.When he drives from behind by lifting his back it helps him to balance and stay lighter on the forhand.
Good luck with your project.

Mastador