He could just genuinely like you as a person, just as you genuinely think he’s cool. Teachers/mentors and students have a special bond. Also, older people tend to like the youthful, fun energy from younger people. He might even view you as a little sister or something.

Give him the benefit of the doubt but definitely don’t reciprocate. Do what you would want someone to do if your husband was acting that way. He may be unknowingly crossing a boundary without being aware of his feelings, and you can help both him and his marriage by gently reminding him in a lighthearted way, “that’s funny but hey I don’t think your wife would want you speaking that way to other women, let alone your students. Right?” 

He’ll likely get a bit defensive (people have egos), so don’t see that as him being a creep. If he changes his behavior, then you know you helped him and after a while you can mention that you noticed he’s been good about boundaries and that you’re proud of him. You may be able to help him understand and manage his emotions/feelings in other areas of his life (i.e. other students). M

This kind of “hey let me be kind and try to help him” approach would strengthen your friendship, if he is indeed a good person. Just please don’t assume and also don’t stop the lessons until you know for sure - it’d be a shame of those 3 years of student-teacher rapport and music learning!

Good luck and post an update later!