Have you discussed with your veterinarian to rule out the possibility of a deficiency? If that is ruled out, he may just be a late bloomer. I agree with you on not loading him up on grain. Better to have a sound horse..no horse show is worth a lame horse. Like people, horses build muscle over a period of time. I would have him exercise like the human athlete’s do, one day on, one day off, giving his muscles a chance to build. If you do this already, you may just have to wait till he’s ready. Some horses just don’t mature as fast as others.