Here is a contrarian thought: why not go to a research university where your son can take graduate courses (not the same as working on a research project!) by sophomore year, and IF your son finds out that a Phyiscs PhD is not the right thing for him -- there are myriad of good reasons why STEM PhDs switch out -- better find out in college than in grad school, or mid-career! It is not just for money that many Physics PhDs work in hedge funds. On the other hand, if he STILL wants to do it, having done PhD-level work early, his chances of success would be far greater.
I believe this is why quantitatively, top research universities (Ivies, MIT, Stanford, Chicago) graduate far fewer future PhDs than cuddling LACs (something that is touted to no end by LAC boosters on CC), while at the same time, qualitatively, the top of the academic fields still tend ot be dominated by graduates from top research universities who did stick it out.