I have some bad news - unfortunately most of the clubs on Pleasure Island at Downtown Disney are closing on 28th September this year They are knocking most of them down to make way for more family-orientated stuff as far as i know
Previously though, i think you had to show ID at the entrance to the clubs in Downtown Disney (we certainly had to last year) and they give you a wrist band to wear in the club which means you are over 21 and they can serve you at the bar (So they dont need to continually ask you for ID all night).

Just got some info from the Walt Disney World website which says guests must be 21 years of age or older with acceptable forms of identification to enter BET SoundStage Club, 8TRAX, Mannequins Dance Palace and Motion. Guests under the age of 18 must be accompanied by a person 21 years of age or older to enter Adventurers Club and Comedy Warehouse - so your son will be able to go into the Adventurers Club and Comedy Warehouse if they are still open in October, but i doubt it I think Raglan Road is staying open, but thats about it.

Ive never been to City Walk, so im not sure how they work it!