When I was studying French at university I went off aged 19 on my compulsory third year abroad to teach as an assistant. We were told at sessions before going, and my experience confirmed it absolutely, that the first 3 months would be really exhausting and we'd be very homesick, we'd all be dying to go home at Christmas, and then after Christmas we'd grow to love it and not want to leave.
With a few exceptions, that was absolutely right. My French was very good before I left, I knew the country, and the region I was in, well, and I still found it very, very hard. It took 3 months to get used to everything - and that was at 19, not 12.
I'd agree that you'd need to look outside the state education system (and that includes most private schools, for reasons stated elsewhere on the thread), as French schools are not geared up at all for this kind of thing, a French internat bears no resemblance whatsoever to a private boarding school, many if not most or even all kids go home for the weekend, and he'd have little to no chance of understanding any lessons; nor would a teacher in a class of 35 pupils be able to give him extra help. He'd not be doing philosophy, that is only in Terminale, but maths, science, history/geography etc. would be very hard. I have been teaching/lecturing in France for over 15 years.