Your son is right that it usually the summers after 2nd & 3rd year. Our one did a (paid) research project on campus over the summer after 1st year, and then did the 2 REUs.
Have your son check his college, but many just want a major declaration by the end of 2nd year- and let you add majors and minors right up to something like 10 minutes before graduation (I exaggerate, but not by much- one of ours added a major autumn of senior year). If physics is the core interest, he can just sign up as a physics major, and take math & cs courses as suits, and if they are adding up to a ‘major’, register for it then (one advantage: you don’t have to take some of the dud classes that every major seems to have stuck in someplace). Grad schools will look at the actual classes you have taken, and are much less bothered by the name of your major(s) than UGs ever believe!