His style of shooting can dictate his draw length also. I'm 5'8" and draw 26" but if I used an Olympic style of shooting I would draw at least 28". Instead I use the G. Fred Asbel style of instinctive shooting which is a more fluid style.
You could take him shootring and when he is comfortable with shooting a bow measure his draw length.
I'm 6'2" and I only draw 28" shooting instinctive style, but you should probably plan on a 30" draw, in case he uses a more upright, wide open style. Compound shooters are usually taught that way, with the bow vertical to align the sight pins. If you put an unstrung bow in his hand, with an arrow on the shelf and held to his anchor point, you can measure that as a starting point, and tiller the bow out to that measurement for safety. In reality, the compression on his shoulders will take a couple inches of that measurement. For example: When I am measured that way, my draw is 30". When I actually draw a 50# bow, my draw length measures 28". When I make a bow for myself, I aim for 50# at 28", but I make sure to tiller it out to 30" for safety, in case I get excited and overdraw. Hope this helps.
Julian