My kid with ASD, or officially PDD(NOS) -pervasive developmental delay (not otherwise specified)-, is TOTALLY into numbers and math. What I've learned is that, for him, and a lot of "spectrum kids", numbers make sense in a way that most of the rest of their world does not. Numbers do what they are supposed to do. Numbers follow the rules. People, social situations, art, imaginary play, etc. are more unpredictable and there are far more variables to consider. Many times people don't do or say things that make sense. The chaos of the playground is disconcerting. Humor is confusing. And ART?! - impossibly subjective. Numbers can be lined up and summed up. They create sequences and patterns.
2+2 always equals 4.
When the world gets too confusing and frustrating, and he's tired of trying to figure out what it is everyone wants of him, he can go to his room and get out a math workbook and find order and solace in the predictability of numbers.