I don't know how long you've been driving, but once he's comfortable, and moves out well, start half-checking him up. Take your little finger and on one side at a time, gently pull and release the rein. If he slows, anticipating a whoa, tap him lightly with a whip and give him the command to trot. Using alternate halfchecks should start collecting him.
Another option is to use draw reins to bring his head in. The best thing would be to find a trainer in your area to learn how to hook them up and use them. They can be very severe if used incorrectly. Basically, the draw rein is a long line with a snap hook on the end, and a slide pull with a snap to hook to the bridle.
If you have a bitting rig, use that, but if not, just use the surcingle from your harness. Harness up the horse with the surcingle and bridle, with out the checkrein to start. The end of the rein can be clipped to one of the side rings of the bitting rig, or the harness if it has them. You can use a buckle to hook to if nothing else. So the rein goes from the surcingle, back to the pulley on the bridle, and then back through the rings on the surcingle.
Then start ground driving him, and pulling his head back carefully while driving him on with your voice and whip. Work 15-20 minutes at a time, alternating directions. Just remember you have a tremendous lever at work, and be careful not to apply too much pressure.
Good Luck!