I would do lots of transitions and make sure to get him sharp of your leg. Also do loads of lateral work such as leg yielding and shoulder in/fore. This has helped my lad tremendously. He is very developed in his neck and shoulder area and his back end is not as strong yet and has a tendency to go on the forehand. Also he found medium trot and any other lenghtening very difficult because of this. Now that I do loads of transitions and half halts he is much improved. I also slowed his trot down to give him a chance to carry himself. Would do lots of work on a large circle and make the circle bigger and smaller through legyielding and do lots of tempo changes. Has really worked for any horse I've worked on.
As above I have also been doing trot to halt, and halt to trot transitions with a few of ours, this really seems to have helped, especially with a large young Irish mare who is built bum high and naturally on her forehand she now seems alot lighter and pushes from behind