I would try to talk the kid out of it, but at the end of the day it is his choice. There are worse things out there then being a bodybuilder although few come to mind. Hopefully I would have raised him correctly so he has better options and won't bother.
He isn't the most coordinated kid out there so high school sports is as far as he likely gets. No scholarships. He is 6 now, and I have him doing running, climbing, hanging on bars type of stuff. We do push ups super setting with wheelbarrow, and he wrestles with dad. I also have a 20 lbs sack of rice he does some exercises with. Thin kid but very strong, and he loves the physical training. When he is 10 we will start the weights.