If he has 24mm from top race a 30 mm will do as you don't want top of steerer above stem and need bit below top to get it fitted you could get away with with a thicker one if stuck as long as the top caps expander is inside steerer ok(if you had 20mm and went with a 40+ thick stem you could have problem's but 10 mmish +/-is ok from top of steerer tube to top of stem.
The main thing is that it's above top line of stem line ie where it goes to bars plus being low/on top of top crown less leverage on steerer tube.
The only time it's a problem if your jumping/pulling up on bars type of thing but then you'd not be using ali/carbon steerer's.
(even if you can understand it)
Give a man a gun, and he can rob a bank.
Give a man a bank, and he can rob everybody.