I agree with everyone. Puberty is a good time to start as far as safety goes, plus you need to remember, at 10 1/2 years old, most kids do not have the emotional maturity to be consistent and give 100% of themselves to training, and there is nothing wrong with that, they're just little kids.
I would say, like everyone else, bodyweight exercises, stretching, running, etc. and save the weights until he is 13 or 14. I personally think the summer before 9th grade is a perfect time to start weightlifting,