I am also a math person who was unsure what tools to give a child determined to drive forward in math, and concerned about their moving forward without sufficiently strong foundations. We found this book
Help Your Kids With Math which has been at just the right level and depth of coverage for my child to explore on their own.
There is a lot outside the traditional curriculum that can be explored and enjoyed. Books we enjoy that are more "off-curriculum" include:
You could also look into math-oriented games such as Set, Spot It, Tenzi, etc.
As someone who loved math as a child I would encourage you not to force your child to slow down, but rather to broaden their view and explore topics beyond the usual curriculum. If it's helpful, my approach has been to provide fuel for my child's passion but not to actively push the accelerator. Typically I will get a new book and just leave it somewhere - it's usually not long before it is discovered and devoured. If your son meaningfully outpaces his classmates and sustains his pace over time perhaps you could explore options for in-school math enrichment and acceleration?
