A couple of ideas for you. First, the leagues that only allow kids 8 and over... We've run into this several times. I contact whoever is in charge, try and explain the situation with DS in as few words as possible, ask for a chance and promise to leave if there are any issues with his behavior. I have only been denied once and we've never had to leave.
As far as mentors go, my DS has a mentor for fractals that he met while on a tour at the local university. As part of the tour the kids were taken through a "mini dome" that is used by the college students to design planetarium shows for the local planetarium. They were then taken to a computer lab and given simple instructions on how the shapes are designed. My son fell instantly in love with the whole "fractal process". The professor giving the tour gave my DS alot of extra time and when we were leaving offered his card and access for my son to the university fractal generator. The two have been pals ever since.
I guess the point is that you can never be sure where a mentor will pop up. Pay attention when people you meet offer up their careers, you may need them someday!
