Our BO has had her son on a a shetland pony since before he could walk, and he was pretty much carried around on horseback constantly from six months to two years. He would sit in the saddle in front of his brother or dad and walk/trot/canter right along with them. That developed his seat very early, and by two years old he was riding his pony by himself in the roundpen at a walk. At three he was walking and trotting, and now he's four he is riding his older brother's 13hh pony alone in barrel races, pole pending, and pretty much doing everything his family does, but on a well-versed, bombproof little mare ^^ sure is a cutie. I really think it depends with your son though, on how well he can balance and control his horse. Andy (the little boy) wasn't allowed to ride with a bridle until he could control his horse with mostly leg in just a halter, so he wouldn't hurt his pony's mouth. Just go with what feels right!