He will thicken and grow more as he continues to mature. I have never, ever, ever seen a horse that did not grow from their 2 year old year to their 4 year old year. My bet is that he will be taller, thicker, and stronger next year, then slowly taper off - my mare grew a hand from the spring of her three year old year til the winter of her four year old year, and my bet is he'll level off completely as well.
It's basically the difference between a 16/17 year old teenager and a 25 year old. HUGE difference. He's just not done growing yet anymore than a teenager is.
Good food helps (obviously). Being out and walking in the pasture also helps her bone density. Keep the great care going - you are doing a fantastic job and he is super cute.
There is no passion to be found playing small - in settling for a life that is less than the one you are capable of living. - Nelson Mandela