Check out EasyJet and Ryanair
There is some very good advice in these posts.
I'm not in the Europe and not a commercial pilot. In the US the market has changed dramatically. I cannot believe the offers high school kids are getting for Cadet programs. A friends 25 yr old brother just started at Delta main line. He went to Auburn. But it comes and goes.
Getting a first class medical and then a PPL would be a good start. I've known a few very healthy people who could not get the medical for assorted reasons.
Really dig and find out what airlines offer training. Ryanair it is not popular on here but my understanding is they offer a cadet program. From what I have heard you can do very well with it. What ever you get , read the fine print and ask a lot of questions.
That your son speaks French and English will make him attractive to some companies. One observation I have is people who only speak English tend to have a view of the world that is not really in sync with what the rest of the world.
The best piece of advice is to have a Plan B.
Personally, mine are grown but I would have pressed them very hard to get a degree or another qualification first if flying was the way they wanted to go. My daughter left the Air Force academy after two years, finished in Engineering , worked a bit in consulting then ended up with a PhD in aeronautical eng. and works at NASA. You never know what will happen.
I would be very wary of what flight schools tell you. As for financing flight training , I would go the modular route , take it bit by bit and be realistic.
France has a lot of flying clubs that offer a low cost intro to flying. Has your son checked any out?