yeah wait intill 15 to start him weight training..
when i was that age I was wrestling, baseball, climbing trees, walking, biking... and the typical sports kids do around a crowded neighborhood with other kids my age. and phys ed in school one hour per day everyday...we did all kind of stuff climbing high ropes, gymnastics, hockey, much more I forget.
if he doesn't have phys ed or peers his age to play with at his age get him to join a sports league football season and baseball season or basketball season. Normal activities are plenty at that age and will keep his muscles tone. i have a huge left rear delt, and and a small right one from playing baseball hours and years on end and a strong right arm/shoulder from pitchingand throwing.