Goodness, what a reaction. Tbh I'd probably go for the summer holiday course approach as well, but if your ds is keen on teh boarding option, then it might work for him.
What you are suggesting is not unusual for German families wanting to send their child to a UK boarding school, though to be fair they are usually sending older dc and often for an academic year or more. There are whole consultancies doing healthy business on advising parents which school to choose and how to go about it.
Might there be similar structures in France, who might have an idea about the equivalent in the French system? You could start with the Alliance Francaise, perhaps, or with Allef, though they are quite an extreme unique organisation who may not be that helpful if you're not signing up to their own programme. Worth a call though, you might be lucky.
Cactus Languages seem to be a respected organisation that do summer language courses for kids, so they might have suggestions?
Thinking about it there are some UK prep schools that offer a term in France, albeit with the normal school cohort, so they might be worth approaching? Also the Good Schools Guide consultancy.