I have a DS5 that is passionate about Math as well. He has a math mentor type person at school and is going to start a local math class for gifted kids in a few weeks on Saturdays. Also, not sure if you have math olympiads at your school but that would be fun, I think that is generally grades 4-8...but they may make exceptions. Our school requested that DS start it next year but I am not sure yet. I also want to recommend that you look into fun Math books. My DS loves the Penrose books (here is a link for the first one):
http://www.amazon.com/Adventures-Pe...mp;s=books&qid=1264692040&sr=8-1Also, just books about Math and proportions are fun. We found this at our library recently....and a bunch of others that I can't recallt the name of. There are so many cool math books and stories out there.
http://www.amazon.com/Very-Improbab...mp;s=books&qid=1264692098&sr=1-1I have heard the murderous math books are neat but haven't gotten those yet, and the life of Fred books.
Good luck, I enjoy math but I never thought Math would be such a main focus in my house. : )