Jamiesilo, the sizing guides are good for what they are, there’s no substitute for swinging a leg over the bike in question.
Find out how tall he is, if he’s smack in the middle of a model size, all well and good. If he’s inbetween sizes, and if he has long legs go for the larger size, if he’s inbetween and has short legs go for the smaller size and add 10mm to the stem length.
As long as he’s not some midget torso’d, giant legged orangutang armed freak, like I am, he’ll be fine.
Simon