I thought the modern theory is that epiphysitis is caused by a nutritional imbalance, not excess calories or protein as previously thought. (Try Googling it). I know I did some research on it one time because I am raising my first foal.
I'm not so sure a vitamin/mineral supplement wouldn't be a good idea. Nothing major, just something to make sure he isn't lacking any vital nutrients. I've been giving my guy ShoGlo by Manna Pro. Not the most expensive thing in world and it can be bought at Walmart. I put a couple scoops into 1/2 a small coffee can of Purina Strategy or oats.
My guy has always been big, from the time he was born, and knock on wood, no epiphysitis issues and he is almost 2 years old now.
I used to feed about a 50/50 mix of alfalfa/bermuda hay, but unfortunately my feed store went out of business so I am feeding straight alfalfa. No pasture grass because we don't have any. I think a 50/50 grass/alfalfa mix is pretty close to ideal.
You might want to make sure he at least has some grass hay in front of him most of the time so he has something to chew on. Otherwise they can take to eating dirt.