Hi Pitchounette,
I am spanish living in England, married to a scottsman, and we have 2 DD, DD1 is 6 and DD2 is 2. My mothtertongue is catalan, and I have been speaking to my DDs catalan since the day they were born, and last week, was the first time my DD1 spoke to me in catalan. They are spending the school holidays in Spain with my family, and DD1 goes to a summer camp in the mornings and DD2 goes to a nursery.
For me, even though is very hard for me to leave my DD2 for the summer, I know is the best for them to learn the language, as they have to speak catalan if they want to communicate. Your son knows, as do my daughters, that you understand english, so he doesn't need to speak french. I am not saying that you should stop talking in french to him, you should keep on using french, but if you force him to repeat the correct french sentence, he might just refuse to speak it all together.
My DD1, when she is tired, she always asks me to speak to her in english, but my answer is always, "no, you know I speak to you in catalan" withouht un upset voice, and then I repeat whatever I was saying, but I don't force her to speak to me in catalan. An now, she is even "teaching2 herself spanish!!!, yesterday, she was telling me the numbers in spanish, up to 30!!. When we go this friday back to Spain, I will give her a treat. ( she doesn't know about the treat)