Hi there! You don't state your own driving experience or if you have other horses with this problem, so forgive me for covering some things you may already know. First of all, for a driving horse contact is EVERYTHING. Many times I've seen others ask this question, they get all the complicated, detailed answers, then we see pictures and the driver is letting the horse slop along without any support at all and wondering why the horse isn't tracking up or extending properly. The first and most important element for helping your horse track up is to support him! Your contact on the reins is what helps him learn to recycle his energy and hold himself properly, shifting his weight to the rear.
Secondly, the horse needs to have impulsion. That means energy and as Susanne said, that begins at the walk. They don't get to just mosey along, the horse must walk out at all times like they have somewhere to be and a sense of urgency in getting there. It may take the horse awhile to understand this if they're used to being lazy but if you insist on a forward walk and praise them for making an effort they'll soon figure it out. You'll be amazed how a good walk helps the trot, the halt, and every other element of their work.
Third is the bend. A horse must learn to soften and bend laterally before they can stretch longitudinally. If you get him to soften his jaw and reach for the bit his topline will loosen and combined with that impulsion we talked about, you should see his back start to swing very nicely and his hind legs will stride up much further under him.
Once the horse has relaxation and energy doing gradually more frequent transitions (walk to trot, trot to walk, walk to whoa, whoa to trot, etc.) will help him learn to shift his weight rearward and go forward with power but be careful to keep your transitions smooth and soft and progress slowly into this exercise as his mental capacity to do it calmly and correctly grows. Don't ask a green horse to suddenly start doing sliding halts from trot or walk to canter transitions! You'll only stress him and lose the relaxation and stretch which allow him to stride under himself.
Leia