Though, @MYOS1634, correct me if I’m wrong, but the prepas and grand ecoles are all tiny. All about the size of LACs except specialized (so like undergrad Caltech, Babson, etc.) Just yes, in general, the relevant comparison of US colleges is to all of Europe. Academic talent in the US runs very deep (we have something like half the world’s top 50 and 100 unis with heavily weighting towards the top; something like that).
But I agree with other posters that you may be concentrating on the wrong thing. For someone interested in medicine, you should be spending a lot more time researching the financial aspects (costs, scholarships, your means) rather than fairly immaterial aspects of his HS career. As medicine is grad school only in the US (though he could try to get in to those fast-track and guaranteed admission medical programs, though there, they would look for potential as a doctor), undergrad is the big leagues, not HS. The US is not France (or many other countries), where how you do before undergrad could determine the course of your life. You grew up in the US, right?