Steven,
The main issue is language rather than the different education systems, in that if it takes two years for your son to obtain the fluency of his age equivalent French peer group, can he then catch up on the syllabus in time, bearing in mind the French State is not obligated to educate your son after he is 16. For many teenagers, it does little good to your self esteem to have to re-take a school year, because you are struggling with language competency.
How is your son doing at school now? If he is happy, settled and doing well, do you really want to disrupt that?
If you can afford an English speaking international school, then the risks are that much less, but even then you should avoid your son changing schools once he has started his GCSE's.
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