It's possible your horse isn't getting enough quality protein and that's inhibiting proper muscle development. Lysine is an amino acid that is often lacking. You can find lysine supplements, but a good ration balancer (like the one I use, Triple Crown 30%) often has enough in it (combined with good quality hay or grass). Here's an article about protein & lysine: Beyond protein basics
You also have to not just exercise your horse, but exercise him in a way that gets him rounding and lifting his back to build topline.