Thanks for the replies everyone. This woman showed me how to measure them today to see how tall they will get. I measured him at 16.2 hands- that seems a little big though, so I must have done it wrong (measured from the joint in the middle of the knee, to the top of the hoof) He was 15.2 when I got him a month ago, and in the past month he has grown like a weed (back end has gone up approx an inch). I guess now that he's getting some good food, his body is trying to catch up in growth.
He's gained quite a bit of weight in the last month, but still has a long way to go.