Dear Susanne, probably nobody answered because nobody knows, LOL. He never mentions the scar on his jaw in interviews. But I guess, and that´s just my opinion, since he had severe acne spots and bad skin as a youngster, that it´s just the product of juvenile acne gone wrong.

For all who don't understand the german language:

Susanne asks where the cicatrice comes from, which Nicolas has at his neck.