11-yr-old’s Ultimate Goal: phd Theoretical Mathematics

[Carnation]

Once he passes Calculus, I would look for mentors for him at your local university where he could even audit courses. 
I know of several gifted math or physics students who got full rides to college because of their advanced knowledge in coursework. I also know a few where the university just picked the student up as a graded student offering free tuition once they prove themselves in the classroom... good PR for the college. 
You just have to be ready for the publicity. 
Once the story of your son is out, you have to be ready for everyone and their brother giving you advice on how to raise your child. Publicity is a double edged sword. 
We have thus far been able to steer away from the publicity but my son1 switched from being math/science kid to history/lit kid which made the avoidance of publicity easier.
Another note, if your son is not a Davidson Young Scholar, you probably want to get your son tested for the program. 
It it free once you qualify and will help tremendously in finding mentors as well as it will open doors for your son.

Best wishes.