Up his grains and pellets... Hay contributes to belly, grains to topline. My ancient pony was completely devoid of any fat or muscle across his topline when I got him - his old owners attributed it to age. I realized that he wasn't able to eat hay very well so put him on a alfalfa pellets and senior feed mash. His topline has filled back out nicely and he looks great. He's very old so no workouts for him... just good food and pasture turnout.
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