Ania,
My 11 year old is also very gifted in math. Luckily, my husband is a Professor in Biostatistics and I have a PhD in Engineering with a minor in Math/Sci Education. We supplement all of our kids' education through our daily interactions and having materials available, but don't have them "practice" math like you would practice an instrument. I do try to say "yes" when they ask me to play a game with them.
If you have the resources, I would recommend buying the kits from a school supply store to help him explore math with manipulatives. We bought a double balance scale, for example, for the kids (we have 3) to explore negative numbers and algebra. We also got the multiplication as area kit that teaches polynomials with plastic squares and rectangles. We often ask him questions, like "How does an octopus do math?" to get him interested in different bases (8 tenticles, therefore base 8). He loved Camp Invention, sponsored by the National Inventors Hall of Fame. Macalester University's Math and CS Dept has a "problem of the week" that gets us thinking. Some of our favorite games are Set, Sequence, Logic Links, Chemisty kits.
I often ask the kids to explain why they think something is true and to generalize it to further situations if they can. I think learning about Bloom's taxonomy and the kinds of questions which are in the Analysis, Synthesis, and Evaluation levels really equipped me to engage them in deeper thinking.
Sorry to rattle on. I'm feeling particularly isolated today, living in one of the bottom 3 states in education.
Peace and Joy,
Alisha
