We have had some of everything over the years, AQHA, Thoroughbred, Paint, Holstein, Morgan even some nice grade horses. Many do not start to fill out until they are 5 or 6, some later than that. We have a palamino paint gelding that just turned 6 and grew 2" in the last 18 months. He's teetering again. My daughter has a beautiful thoroughbred mare that was bred on a polo farm, she washed out and now she is trainig as a Triple A barrel/gymkana horse. She started her last year when she was 7. Go slowly, take it easy they tend to be a bit high strung with lots of energy. Give him time to grow into himself and broaden, let those legs thicken up a bit, he is still very young. That being said the only critism I have is that he isn't square in the pictures of his hind, so its hard to tell if he toes out. His hip is short and with work his butt can be built up(he'll need it for impulsion), his tail is set a little low but I've always liked that in a horse that has to sit back, spin or slide. Muscle is a great equalizer. His back isn't really short and with the long rear pasterns and cannons he should be able to get under himself and have a nice long stride.