In IL most of the enduros use some road sections so you have to be sixteen and have a drivers license. I use to ride enduros when I was 13-14 in Wis because they were all off-road and the club ok'd it. There is a enduro in IA that alot of younger kids because it is also all off-road. I would say your best best is to call the promoting club and ask them if they have an age limit, and let them know that you would ride with him all day. The club may be worried about a younger rider out there all by himself. They may also not let him due to insurance reasons, most enduros are not considered to be an youth event so the insurance may put an age limit on the riders that can enter, most clubs don't think about that part of it.
Hope this helps.