It could be that he has over and/or under developed muscles in places that would proportionally cause him to be heavy on the forehand. He may need some re-training to help him carry himself better and collect, which could take some decent time since he's developed this way of carrying himself-- if it isn't purely a conformational issue.
It may be a conformational problem where he is built to carry more weight on the forehand, which could also be helped with training but may not be necessarily correctable. Does he have any known problems involving joints, tendons, ligaments, etc. in his hind legs or any back problems that may cause him to compensate for pain by avoiding engaging the hind?
I would recommend having a trainer take a look and maybe start him in a program to keep him supple and distribute his weight more evenly. You will be able to feel a difference once he makes progress, and you could try to have a friend record you & him at different times and compare his movement so you can see progress better.
There's some good websites with exercises that you can try. Not sure how you feel about this one but here's a start: Exercises for a Horse Heavy on the Forehand