I stared working out (Heavy lifting, tons of sit-ups, yadda yadda) for 30min-1 hour a day when I was 12-13. I am 5'11" taller, by alot, than my parents. One thing he shouldn't expect though, as has been said by others, is a six-pack at such a young age. I don't know if it is immpossible but I didn't even have an outline of a six-pack, just REALLY hard abs... I think this has to do with the fact that your body needs to store energy, fat in this case, to make him grow taller. If he tries to get abs at a young age this directly conflict with the fact that humans store growth fat on thier midrif. So the moral: Don't seriously try to get a six-pack till your about 17-up.

On the buying him wieghts, I was lifting 100-150 between ages 12-13 and don't lift as much now so I really couldn't tell you now, but needless to say I am 5' 11" so I really don't think it will affect his height.