To a point I'd just want to get him on a bike, any bike. It has to be safe and you want it to fit well enough that it doesn't turn him off riding.

How many of us have heard "oh, I tried riding once. But _____ hurt and I stopped".

I would not worry about the perfect fit because as he gets fitter, looses weight and/or inches what fits him now will change. The we can obsess with you over dialing it in

I'd think safe, acceptable fit and .... let's go ride!!!!