Like bodyshop20 said. Keep it fun because that will keep him motivated.
The good thing about his joints, muscles, and bones forming...he's NOT going to push far or hard enough to hurt himself. Let him train as hard as he wishes...and modern pediatrician who's kept up with their reading will tell you the same. It will not stunt his growth or damage his bones and joints. Just let him have a good time...
And it's great that you are encouraging him to be fit.
As in the other thread...too many parents are teaching kids to be soft, weak, and fat... yours is on the right track as led by your good example.
My 10 year old is now training for his first powerlifting meet... yes, they have a pre teen division. If there is something physical and active that they enjoy...encourage it and lead with a good example as you've done.