It won't stunt his growth...neither will caffeine...also going out in the cold without a coat will not get him sick and sitting close to the TV will not ruin your eyes.
Buidling mass is such a long term project that your kid, lifiting 3 days a week, won't hurt him at all unless he drops the weight on his toes...the only thing that will stunt his growth is the genes you and his mom gave him. And possibly power lines and nuclear waste...if you live by either of those things.
If you are gonna be short you are gonna be short...if you are gonna be tall you are gonna be tall.