I'm in Canada and homeschooled my kids at that age and stage, so I'm afraid I can't suggest much about the school differentiation issues.
But about what you could be doing with him math-wise that wouldn't necessarily just accelerate him through the school's curriculum, here are some suggestions:
Math / critical thinking puzzles. Brain-teasers. You can often find neat books of these. The story of the seventeen camels is a good example of the type of problem I'm thinking of. A book that has a lot of these in a wonderful presentation is "The Man Who Counted" by Tahan.
Further in the math stories vein ... books like Theoni Pappas' "Penrose the Mathematical Cat" and "Math for Kids and Other People Too," or Hans Enzenberger's "The Number Devil."
Geometry activities: tesselations (there are some neat apps, and M.C. Escher's work has always been appealing to my kids, but you can have fun making them by hand with cardboard and paper), weaving and knitting (for the patterns), geometric art with compass and straight-edge (learning to make 90- and 60- and 45-degree angles with just these two tools, making complex mandalas), geometry and patterns in nature (the golden mean, Fibonacci series, etc.), origami (topology), tangrams.
Vi Hart's math-and-art videos are quirky and incredibly fun! The hexaflexagon ones are a great place to start.
Hands-On Equations is a neat system for introducing algebraic problem solving and negative numbers. Should be easily accessible to your ds. My dd did most of the program in KG and 1st.
Music note-reading (especially the rhythmic component) is highly mathematical and pattern-based. If your ds is interested in learning an instrument, the sight-reading will stretch his mind mathematically.
Projects and activities that use graphing and probability typically really entice math-keen kids, yet without directly accelerating their transit through elementary school scope & sequence. Learn to use spreadsheets and create graphs from them. Measure the growth of something. Follow weather measurements. Plot the day length, or the moonrise time. Do a survey and plot the results. Do a random-event experiment and figure out the probabilities.
Learn about scientific notation, and look at how it's used to better understand the scale of things microscopic and astronomical.
Learn about exponents and logarithmic scales and their applicability.
Explore hexadecimal and binary number systems.
Learn about Roman numerals and the importance of zero. Try to do multiplication with Roman numerals.
Explore the relationship between fractions and decimals ... and then learn about irrational numbers.
Learn about fractals, cryptology, compounded interest, investment math.
Do some computer coding and scripting.
Play board games that involve critical thinking, pattern-prediction, computation, visual-spatial planning or probability.
Revisit some of the basic arithmetic he has learned, but with more depth and breadth. For instance, explore finger math, alternative algorithms (using negative numbers rather than regrouping, partial quotient division, lattice multiplication ... and help him learn *why* these all work).
Hope that gives you some ideas!
That is a great list and I am so happy to see that our math teachers use many of these ideas! DD's advanced math class was rewarded for finishing their state tests with a project on tesselation. I need to find those apps.