Yes, he can get into all but 2 of the clubs. (I can't remember what the names of those clubs is.) You will need to be with him, of course.
My recommendation would be to take him early in the evening, before people have gotten too drunk and the really heavy partiers show up. Also, Sunday through Wednesday are the calmest nights. Thursday is when the CMs go, and the locals go on the weekends.
He may want to check out the view in some of the dance clubs, but he'll probably enjoy the Adventurer's Club and Comedy Warehouse the most. The jokes should be appropriate for a 12 year old, but of course you can't make any guarentees about the other guests.