Things to remember....
Bones are still developing at such a young age, and if you build too big and strong for your bones, your bones can actually bend. My mother used to work with a guy like this.... now he's 40, and all buckled. That's also because he only trained his chest and shoulders and neglected his back.
Also, there's some debate about this.... but I stopped growing when I started lifting heavy weights. Most bodybuilders are under the average height....
I think 14 is an okay age, but I would spend a year with him.... with light weight and high reps, getting his technique perfect on every exercise. Maybe even keep him on machines until he's 15 or so....
.... it's up to you, but I'd be careful if he were my brother.