Well Cosmic, opinions differ here and I will give you mine. If your son is having no problems then leave his foreskin alone however much there is. There is no particular age at which you should be able to retract your foreskin, it usually comes by experimentation and the only problem with that is that he will retract it and not be able to get it back.....this is unusual though.As pudducat so eloquantly put it, leave well alone. Of course, if he starts to have infections (Balanitis) then my commentsi n my first answer come into effect.
Your GP might have a differing opinion though.