you got a wacom tablet laying around, mine came with a version of corel painter (or you could download it) which I think would have worked on a mac too, I never bothered checking to be honest as I wasn't going to use it
Corel (draw) is still going but they do some specific apps which might be more to his liking.
Painter (11), painter essentials (4) and Painter Sketchpad - (
links are on this page) - they're all available on mac too

- the first two are more the painter style of work while the sketchpad is more of a designers sketchbook in my opinion.
You also have
alias sketchbook pro (I liked this more personally when I tried it but its individual preference as they're all pretty similar) - probably a mix of the two above.
You also have anime/manga studio and artrage deluxe (all mac options too) -
linky to sales page and they're pretty self explanatory in their names
Theres also some cheaper/free alternatives that might be worth looking at
here
In my opinion the best program depends on what sort of work he wants to produce and how he feels comfortable doing it, might be worth trying some of the demos first