Have you given a try with AOPS' Alcumus for supplemental math (http://www.artofproblemsolving.com/Alcumus/Introduction.php)? I don't know if it is too easy for him, but it looks like it might be around the right level based on what you're saying and it is free.

My kids are not as mathy are yours or Dottie's, but I do have to say that puberty made no difference in my dd13's interests or speed of acquisition of knowledge in her strong areas and passion areas. Sorry wink!

Study Strategies for Accelerated Learners