First thing to rule out is any physical reason why engaging his hind end and tracking up could be uncomfortable for him. Is his saddle properly fitted? Could it be in need of a reflocking? Is it possible that he has an injury from his track days that could bother him a bit at the trot? Has he been seen by an equine chiropractor or massage therapist? He could be "out" or stiff in his back and/or hindquarters. How are his teeth? If his mouth hurts, he's not going to want to work forward toward the bit and use himself fully. It could be lots of things that have very little to do with training.
Do you work with a trainer/coach/instructor? If all of the above checks out, then it is probably a matter of building up the right muscles, and a year is a pretty short timeframe for that to happen even under the best of conditions (i.e., highly experienced trainer aboard, daily workouts, stellar conformation, etc.). If it is simply a matter of training/time, give yourself and your horse that time to build up the muscles to engage better. Enlisting a good coach/instructor/trainer (if you haven't already) can be a great way to fasttrack your success and make sure that you and your fella are on the right track.