I had a Thoroughbred mare, who came to me at 8 years of age with shoes on all 4 feet. Since she was race bred, I'm sure she probably had shoes on from about 2 on up, and after having her for 3 years I ended up pulling her hind shoes after she tore her superficial flexor tendon on her front leg. Since I had bought her, the farrier was pleased with how hard her feet were especially for a TB, and she was fine even after being able to be worked again with only front shoes. Good enough to be able to do trails, and jump 5 foot courses with no problems. The one thing that I would be careful of, is making sure that the shoes fit him properly, and that they are not preventing proper growth of his feet. A very common problem I see in horses is people shoeing them young, 2-4 years of age, when they are still growing, and their feet end up getting messed up because the shoes weren't changed often enough, and correctly, and the feet ended up getting constricted, stayed too small, and the frog and heel got squished, all which results in a horse with soreness issues. So I would just watch for that, since he is still growing, and just make sure that his feet are still developing normally, but I don't see a problem with just the front being shod on him or your mare.