He looks fine. He only needs about 50-75 lbs. He should gain just fine on the hay you have for him. Make sure his teeth are floated and you have him dewormed well.
If you want to give him something extra, I would use alfalfa pellets and some corn oil. Feed him 1 scoop (3qt feed scoop, about 3.5 lbs) of the alfalfa pellets and 1/2 cup of oil. That will put the weight on him quickly and won't make him hot or hyper.
Sweet feed, pelleted feeds, and even senior feeds can change a horse's personality and temperment, negatively. I prefer a natural forage based diet, using hay pellets and vitamin supplements to round out nutritional requirements.
Alfalfa pellets put on weight better than most feeds. They are high in protein, but they are also high in amino acids, which puts on MUSCLE. An underwieght horse needs quality protein and amino acids to put on good muscle weight, not just fluff. 3-5 lbs of Alfalfa a day, by pellet or hay, will not provide "too much" protein for a horse.