Actually, if your son has reached puberty, 13 *** MIGHT *** not be that bad of an age to start learning a curve (if it's taught properly), according to at least a couple of sources I could quickly find. See the ASME web site at
www.asmi.org and click on "USA Baseball Guidelines for Youth Baseball Pitchers". Also, several sports medicine specialists, including Dr. Joe Chandler of the Atlanta Braves, have done studies and research on this. See
http://www.cababaseball.com/CABA05/usabaseballinjury.htmlI found it interesting that the average age at which provessional pitchers started throwing the curve was around age 14.8 - that tells me that those guys must have benefitted from spending plenty of time perfecting their fastball and change.
EDIT - I didn't originally pick up on the fact that your son hasn't pitched before. In that case I'd definitely suggest that he just work on his fastball mechanics first, then when he can consistently throw fastball strikes work on the changeup.