Schools will look at your son’s actual grades. Weighted GPA is computed so differently in different high schools across the US that it is essentially useless. I agree that he should make sure that the transcript that is sent to universities includes his actual grades from the European school that he attended.
You might want to also talk about this with his guidance counselor, and have the guidance counselor make a note of it in his or her recommendation.
Is your son a citizen of the US, France, both, and/or somewhere else?