I was a math major in university, and my first job (a long time ago) was as a software engineer at a physics research facility. I have also more recently had a couple of tours of other major physics research facilities. To me these three majors are very much related. At least both particle accelerators and large radio telescopes use a lot of computers and a lot of mathematics. I would expect that this applies to other physics also but I am not as familiar with other physics research.
I am not sure that I see the point of a triple major. However, I also do not think that your son needs to decide this yet. I at least took the courses required for all three majors through the first two years of university, and then decided to focus primarily on one (math), with a few course in the second (CS). Physics I stopped taking after quantum physics.
At least to me physics becomes less “real” when you get to quantum physics. Your son might want to get this far before he decides what major(s) to continue with.
If your son is interested in physics research, I would be inclined to be more serious towards applied math rather than pure math. At least linear algebra, calculus and differential equations, and probability and statistics and stochastic processes can be useful in physics research. There may be other types of applied math that could also be useful.
I do not know how physics or CS research proposals would be related to topology. Perhaps I am just missing something here.
My suggestion is that your son not worry about it too much, and take classes and participate in research that he finds interesting. Nearly all of us take some time to figure out where we want to focus. It is very common to first make progress in one area and think that is what you want to do, but then discover that something else is really the right path. If you have the brains, background, and drive to make a contribution in one area, then you probably can also make a contribution in any one of multiple different areas.
I would not try to find a physics project that relates to topology. Rather I would try to find a physics project that is interesting, and if reasonably possible uses computers and math (which I think describes a lot of physics research).