I gave up my own searches and it's time to seek help.
Looking for a movie, probably from the eighties or late seventies. Drama which tells the story of a French child, who comes on vacation to stay with his mother's old friend. A married guy. Later on, it revealed that the boy is his own son, creating a large tension with his current wife. I was pretty convinced that the child named Jean-Claude, or similar French name, but many years have passed, and I may be wrong.
Anyone know what I'm talking About?

Was it "Man, Woman, and Child" with Martin Sheen?

It's from 1983.

Yes. My new hero. It suddenly seemed so easy. I know every movie that Martin Shane ever did. At least I thought I knew. Even Jean-Claude was right childs' name. I haven't seen the film for nearly 30 years. Do not understand how I didn't find it searching the Imdb database. I was looking by character name, by keyword and plot. I thought it was a very small budget movie, just came to me by accident when I was a kid. In any case, many, many thanks

You're very welcome. I knew from your description it was the Martin Sheen film but I didn't remember the name. I also remembered the "Jean Claude". Very touching movie. I think I've seen it twice.