Those are all good schools, but if I had to pick, I’d narrow it down to UCSB and UT-Austin. I’d personally prefer UC-Boulder for the location, but I don’t think its physics department is outstandingly renowned. However, realize that the programs are going to be about the same at any of those schools, and the quality of actual education will largely depend on the interest and enthusiasms of the professors. What is most important in terms of getting into a good graduate school and getting a firm grasp for what area of physics he’d like to get into are undergraduate research opportunities; meet with his advisors and the department chair, if possible, and ask about what research opportunities are open to undergraduates. The more opportunties the better, and especially if it is a project affiliated with a national laboratory or across schools (especially international) as he’ll get more opportunities to meet people working in different areas, make contacts, and see what kind of tools and methods are in regular use so he can have familiarity with them once he gets to the graduate level. (Do not rely on just what any school offers as being the standard across all research; they often select tools that have an academic discount, or because some particular professor likes them.).
A few other comments which you may take or leave as you like:
Although your son may think he knows what he is interested in doing, it’s never really a certainty until you spend time doing it. Theoretical physics can seem really interesting and esoteric from the outside, but there is actually a lot of plugging away at things and doing a lot of tedious calculations where a single mistake can negate hours of solving and rearranging equations. Modern computer algebra tools are a real godsend for this, allowing you to revise a mistake and have it fixed through the subsequent work (to a certain extent) but it can be very frustrating and not at all sexy once you get into it. You can’t just like the end result or expect that you’re going to have some physics-altering “Eureka!” moment; you really have to enjoy the process of solving problems. I highly recommend going through Jearl Walker’s The Flying Circus of Physics and try coming up with solutions to the often surprisingly complicated issues arrising from seemingly simple problems to see if he really enjoys the theorectical side of physics.
Mathematics and numerical methods: early, often, and as hard as he can stand. Whatever analysis and visualization tools he ends up using, understanding the fundamentals behind how the operate, and how the basic mathematics works is so critical to getting past concepts that are not innately intuitive (e.g. all of modern physics). Statistics is particularly important; a good theoretical physicist today needs to be as conversant in data science methodology as a dedicated data scientist, and at least friendly if not intimate with statistics, as well as grasping all of the vector and tensor and Hamiltonian methods. He should graduate with a mathematics minor by default.
Compuational tools and methods: learn how to build purpose-made tools from scratch, ideally using several languages or environments. The day that physicists stood at a chalkboard and filled it with equations and diagrams is coming to an end; the solutions for modern problems are just too complicated and too difficult to visualizate and solve by hand. The ability to build simulations and manipulate complex equations with the aid of a computer is crucial, even for someone who works in a purely theoretical area. He doesn’t need to be an expert programmer per se, or write beautiful code, but he should be able to write a program in Python or Julia that can take a particular solution to the Einstein field equations (as an example) and simulate what it actually does.
Networking: you say that your son is happy with just a few friends, but making and maintaining connections with a wide array of people, inside the field and out, is crucial to succeeding in academia and in research. No successful scientists today operate in a vacuum (nor have they really ever, with rare exceptions) and it is important to have people to call upon, both for their technical expertise and constructive criticism, and to cast a wider net when looking for reserach opportunities, especially because it is likely those opportunities will not be in the United States, thanks to a goverment that has made it a priority in the last twenty five years to consistently defund pure science research, from the SSC tand controlled nuclear fusion to stem cell research. (I’m not bitter…I’m not bitter…I’m not bitter…)
Outside interests: It’s college; it’s his first opportunity to meet people outside of the narrow confines of high school, and do things that he has not had the opportunity to do before. Take advanage of both the social and extracurricular opportunities of going to college. Join a club or two; take a trip to Europe for holiday break, get out of his shell and find out what else he might like to do. I personally regret not doing more (although I was working full time while going to school) and in particular not taking more courses outside of the technical curriculum. Take a psyche class to meet girls, join the newspaper staff and write a column on science research, anything to expand his horizons.
Final note: Your son is likely familar with being “the smartest kid in the room” and able to get by in normal classes with only a moderate amount of effort. (I never studied outside of class for any subject thoughout junior high and high school. I literally wrote essays in class, studied physics during English lectures about books I’d read two or three years preivoulsy, and memorized lists of facts for history and geography five minutes before tests. It wasn’t until I took two semesters of university-level calclus my senior year of high school that I actually found any schoolwork challenging, and it was a painful shock of getting the first ‘C’ grade that wasn’t due to just not giving a fuck about the class.) The odds are that he is going to be disabused of that notion in the first difficult class he takes; he may not even be on the upside of the mean. I don’t think there is any way to prepare for this other than to experience it, but he’s going to have to recognize that he’s probably not going to be the genius-kid in that environment, and that’s okay, because learning to put in the hard work to learn difficult subjects is part of the growing up he’ll have to do to be successful in a highly competitive field like theorectical physics.
Good luck to him, and I hope he enjoys whichever school he selects and does well.
Stranger