You are going to run into problems with almost any type of job except agricultural, mostly because of the worker's comp issue. I used to manage a public library across the river in MO, and like most PL's we hired a lot of teen shelvers. We would get IL applicants sometimes who hoped that MO would be less strict on the work rules; we were, but we still didn't hire anyone under 16; our comp policy didn't allow it. Even self-insured municipalities &/or counties usually adhere to this limitation.
I think your best bet is self-employment in some kind of service work. How is he with animals? Lots of folks go on vacation in the summertime so pet-sitting can be a good option. I started doing that when I was 12, all year long, and I made a pretty nice income off it for a kid. If you attend a church the bulletin is a great place to advertise. This does take serious dedication, attention to detail (b/c of needing to lock up peoples' homes) and a willingness to deal with the products of dog-walking, but if he can handle the responsibility, it's a good gig for a teen.