I think if your child is academically talented, there are opportunities through CTY and the like
My daughter has a friend who skipped high school, and entered the university at 14 which was the maximum age for the early entrance program
For some kids- this sort of thing is a relief from "normal" high school http://www.hoagiesgifted.org/
(I mean the CTY stuff- lots of my daughters friends at Reed- did CTY, and they thought Reed was like one big CTY camp- with beer)
BUt please don't think your 13 yr old HAS to be heavily involved in college prep type activities.
My older daughter just graduated from college ( Reed), and she never had any extra academics in the summer, although one year she did do a CTY snail mail course ( but while I thought the instruction was very good, it wasn't needed to supplement her school)
IF he has been involved in camps- summer camps- spring break camps- what sort of actitivies did he like?
I happen to think sports teams- well run, teach a great deal to young people and are a very valuable way to spend some time.
YOu might try local community organizations, but usually they do like people to be older.
BUt some of the local parks have camps that young teens can help with- I can think of several science camps, that have teens entering high school that assist as junior counselors.
My older daughter began volunteering at the zoo, when she was 12, but when it was her sisters turn, you had to be 14 ( and that program is gone now)
I think that would be the first place I look, are programs that he already has shown to enjoy, and see if they could take him on , in more of a helper position. It may not be exactly what you are thinking of, but it would be a way for him to build a small resume, and see different aspects of something he already knows about