On the whole you won't find (m)any in France that do this, however, generally speaking, once you go to the countries that are stereotypically good at English (eg the Nordics and the Dutch-speaking countries) you will find a lot of courses taught in English.
More specifically, I have heard very good things about Flemish (Dutch speaking part of Belgium) universities in this respect. I remember seeing an article about British students studying at the University of Ghent (in the Dutch-speaking half of Belgium) in one of the broadsheets (The Times I think). The University of Leuven is probably also good in this respect, but geographically speaking it is on the 'wrong' side of Brussels. Both these universities have good English language websites.
Sadly, with the fog of Brexit surrounding everything, the cost of your son's education may go up.