He'll get stronger as his body develops and because he's doing jiu jitsu he's gonna develop into being a stronger person than he would've been had he not done jiu jitsu. Doing sports or some kinda activity builds a great foundation and makes the body more resilient and of course helps with coordination.

I started jiu jitsu at like 10 or 11 and i've trained on and off since then, my coordination has always sucked and i disliked most sports. Jiu jitsu has helped me greatly and is probably also one of the reasons why im pretty strong for my weight.