Both Sweetwater Center for the Arts in Sewickley and Pittsburgh Center for the Arts in Shadyside offer photography classes for teens.  You might need to check the required age for classes in each location and what is offered in a particular term/semester, but they are good places to start.  And I'd call to ask if an exception can be made to the required age given his particular interest in this topic.  I think it's currently 16 y.o. at Sweetwater and 14 y.o. at Center for the Arts.  I'd bet that there are photography classes for younger teens/children offered in the summer, though.

Another idea would be to contact a professional photographer and ask if he or she would be willing to offer a one-hour lesson to your son.  Good luck.

I actually am going to 2nd Rachel Shepler, she's worked for us as an assistant quite a bit (my husband is a photographer too) and she does wonderful work.  she would be do a wonderful job teaching your son, she's great with children!