I would say get rid of the plain sweet feed. Get something made for building weight, and I would get alfalfa pellets or cubes and BOSS, to add to the grain.
My mare was losing a little conditioning being on just her hay, which is a grass mix. So I started with adding some alfalfa pellets, and a bit of wheat germ oil, and she has gained what she lost quite quickly, and is actually getting a bit pudgey without being worked as frequently as she was this summer...Lol!
"The ideal horseman has the courage of a lion, the patience of a saint, and the hands of a woman..."