The R-L21 Plus Project has picked up a new Danish member. Interestingly, his most distant known y-dna ancestor had the surname Matrois. I'm no expert on Danish surnames, but that one looks French to me. It's kit 210257, from Nakskov, Denmark, on the island of Nykøbing.
It does indeed look French. How old is his MDKA? Many Danes and other Scandinavians have in more recent generations adopted unusual surnames to distinguish themselves from the thousands of others with the same patronymic surname (Hansen, Petersen, Olsen, Larsen, etc). Presumably if his ancestry is French, he will be aware of it. I suggest asking him.