He is very cute. I agree with all the above. If he hasn't really been jumped much, and its been small stuff, there shouldn't be any issues later on joint wise, as long as you give him time to let him finish growing. Right now the focus should definately be on balance, and knowing where his feet are, and where the rider is. I have a friend who couldn't start jumping her horse til she was about 7 because she was so unbalanced, especially as a slow growing warmblood, that she needed those extra years of good flat work to really be able to find out where her feet were. She's doing awesome now, about 5 years down the road, and has no joint issues. Waiting another couple of years before you do much if any jumping is in the horse's best interest. I know someone around here who sells horses that at 3 and 4 years of age are theoretically jumping 3' courses. I've known a couple from 4 or 5 years ago, and they have some major joint issues now, because of early jumping. I'm glad to see that you are concerned, and are wanting to do what is best for the horse.