Ania,
I think you've mentioned this one, but in case not: http://www.artofproblemsolving.com/

I've also heard good things about MathPath camp: http://www.mathpath.org/

Here's a review of a book you may find interesting: http://www.davidsongifted.org/db/Articles_id_10281.aspx

There are also contests. Although I have a son close in age, it seems that our parenting styles are quite different, so I can't give you any advice. In our house it works better for DS10 to get Math at his level at school (a class aimed for 13 year olds) and let his hometime be for homework, music and diversions. I think that your way has many, many advantages, but it isn't in me to pull if off.
((hugs))
Trinity