As a public school math teacher, I have a couple suggestions to consider. First, I think that it is foolish to consider slowing a child down. I think whatever their interest and ability, encourage them to continue learning all that they can. There is plenty of math available to learn and many others have offered good suggestions.
One caution that I would offer is that you try to set up your courses so that in his senior year he studies either Calculus AB or Calculus BC or both. I have seen students who complete these courses prior to their senior year, and too many of them end up taking the senior year off as far as math goes. I think that is a mistake.
If I were in your shoes, I would let him continue at a natural pace until he finishes Algebra 2 (that would be at the end of 8th grade, I assume). At that point, you have 4 years to complete Trigonometry, Pre-Calculus, Calculus ABC. I would consider in 9th or 10th grade doing some computer programming (I would check out HTDP.org if you want a free option that is pretty good) and/or Statistics. Perhaps you might find a Discrete Math course that you could do his junior year.
In other words, let him work as fast as he can/wants to. Add depth to the curriculum in grades 9-11. Have him complete pre-Calc in 11th and Calculus in 12th.
One opinion to add to many others.
Mike
aleksmath@gmail.com