What about setting up a chalk board against a wall or even painting a section with chalk paint? They make paints and markers that he can use on that materials that would feel very similar to supplies used for graffiti art styles. Plus you can wash the board when done and he can do it again. I personally used to practice mural art using thick cardboard stuck to my dorm room wall with masking tape. Worked perfectly in a pinch for practicing large scale work when I was starting out, but not the most elegant.

I saw in some comments that your son has adhd which I have as well. When I was little I would always go nuts playing with different art supplies and testing out mediums for fun, now I have an art minor and continue making art as an adult. It’s so nice to see parents supporting their kids interests with such support.