You don't want a skinny No 8 (feed him up!!). Does he run fast? If so he could be a centre, full back (you mentioned he could catch) or on the wing if he's fast.
Mind you, if he's got a little bit of aggression, No8 could be ideal!
A friend has a 14 year old who is already 6'00", with 11 1/2 feet, and he plays prop!