Wis75 made an excellent point. Undergraduate degrees do include a breadth of requirements. While it is really exciting to think about all of schools with excellent math programs, a college major contains a relatively small number of credit hours relative to the number of hours to graduate and requirements in all areas must be satisfied. When looking at college catalogs, don’t just turn to the pages on math because those classes/options are already enticing. Instead, look at the general requirements and talk with your son about all the kinds of classes he may encounter. Are there areas with core requirements that are interesting and some that are personally yucky. Would he prefer psychology over sociology? If so, would a class in experimental psychology be more appealing. I would start talking to your son from the very beginning that he needs to limit his math coursework to perhaps one class per term. Just this idea may start getting him to focus on other requirements beyond math. What would be ultimately very distressing is your son taking all the math classes and doing extremely well, but not have enough credits in other requirements to cobble together a degree. Even if it could be possible to substitute math classes for general requirements, that does not give him the broader world view in which to locate math socially and ethically.
Practically too, assuming he is likely to earn a Ph. D., he still has to meet admissions criteria and relevant standardized tests. He needs to be beyond his high school understanding and level in reading, writing, and other areas for the admissions tests. Graduate classes are very often interesting and no more challenging than college-level classes so I wouldn’t worry about them.
Writing, even when the dissertation is short because of the topic or field, challenges many qualified students who end up ABD, Depending on the field in terms of percentage, a huge number of individuals don’t finish because of life challenges like money and time as well as more clearly academic concerns such as writing and data analysis–drawing and writing conclusions within the constraints of data.
Issues with executive functioning and organization could have an important impact on writing a dissertation. I found the dissertation no big deal. However, I spent hours reading and thinking and sorting through information to hone in on a topic and then to flesh out what I wanted to do and how to do it. I had my own little cubby at the university to work in, but it was only me and my interest in working independently in that space that got me through. Faculty and other grad students are interested in what you are doing and may provide tips, but you don’t have a mentor keeping tabs on you and your progress.
Final point on the dissertation or other papers and theses in grad school: After your earn your degree you are free to pursue whatever you want. The dissertation is only a hurdle. If your committee chairs, says type in green on peach paper (not literally this example, of course), do it. If s/he says concentrate on this aspect of your topic, do it. Whatever, do it. Your goal is graduation and nobody is much interested in your dissertation after you are done. You don’t need to alienate any support system that may come your way.
I hope all works out for your son. If he is Aspie, he might look at Temple Grandin as an example of a highly educated person with Aspergers. She earned a doctorate at Illinois and now teaches at Colorado State. She has mastered independent learning, teaching, speaking and writing to the degree that she completed a dissertation and was appointed faculty (hired) at an excellent school.