Thanks everyone for the advice. So far I've been giving him free choice grass hay, as well as a flake of second cut alfalfa once a day (that we feed our sheep). I've also been giving him a cup of soaked alfalfa cubes twice a day -- I forgot to mention that this was also part of his feed at his previous home. He was dewormed the week before I picked him up according to his previous owner.
He is getting free-choice salt and Equine Merit horse mineral supplement.
In two weeks or so, we will move our five horses to their summer pasture, and they will be there until November.
His sire is 17.1 and his dam is 17 hands. Do you guys think he's on track for this height? This is the first baby of this breeding and potential size that I've raised.