Personally, I would wait until he is a bit older - say 13 or so, to start with a regiment of weights. For now I would stick with light weights, chin ups, sit ups and so on. That still builds and defines but does not give the muscles a 'memory' and will allow him to grow as normally as he would naturally.
On the flip side, I have a friend who is a pro weightlifter who had his son in with him from age 6. The son is about 16 now, deeply defined, strong as an ox but to me, a bit too thick to appear 'normal'. He also seems to have stopped gaining height - which may be a function of his genes and not his lifting.
Your son wants to do what daddy does and lifting sends spurts of endorphines and just feels good. Have him train against his own body weight for now and take up running. Waiting a few years will feel like forever but...
dais my 2 cents as an ex-semi pro.
