None of us knows anything about OP’s son other than his acceptances. Based on his acceptances alone, I’d guess that he is likely unhooked and an exceptional student. Stanford isn’t a school known for tough grading. Grades may be less inflated at the other two schools, but getting top grades isn’t as difficult as they used to be. My son chose one of the most challenging schools a few years ago when he had a similar decision to make. My spouse had some concern, but I knew my son and the school well enough that I agreed with his decision. It turned out that he never had a single A- (which he had a few when he was in high school) and graduated at the top of his class. Only OP know their son well enough to determine if a more challenging environment is appropriate for their son.