If you are going to use more advanced coaching techniques such as foul line drills and I assume some one and two step drills, (combined with his age) then I would just jump to fingertip.
If he was younger, 4-8 or so, then I would probably suggest conventional or somewhere in between. But as a 13 year old, he will want to see the things other kids his age, and you, are doing and replicate it.