Just be careful if you decide to grain your horse (complete feeds like the 'mare and foal cycle' feeds included). Any feed that has extra protein in it you have to be careful since it can effect their tendons (ive seen it happen, its not pretty). Actually, if you have a mixed hay (alfalfa, clover, grass mix) there are all the vitamins and minerals they need besided salt which they get from a salt block. If you want to give him minerals, I'd suggest Dr.Reeds. You can buy a big bag and it lasts a long time. You could also try feeding him beat pulp, this works with some horses. My friend has an arab and she feeds him beat pulp with a glug of veggie oil and he put on weight quite fast.
Other than that, make sure his deworming is on track, and make sure he's getting his fill to eat, make sure he's getting to the feeder without being chased away. Some horses are just thinner when they're young. We have 4 2 year olds at the barn. The 2 warmbloods are on the slim side, the QH cross is normal, and the draft cross is FAT. And they're all eating the exact same thing. The 2 warmbloods are at the feeder more often too. So yeah, sometimes there isn;t a lot you can do until they stop growing and you can start graining them.