Which Univ. are you in? If the prof dons't allow you to sit in, just choose another one. I don't think this prof is a good guy. As I know almost every prof. will feel happy if you are interested in his lecture or reasearch. I audited a class last semester. The prof not only grading my homework but also write a lot of comments in it. Sometime, i even feel sorry for my handwriting.
As kong zi said " You Jiao Wu Lei", a great tutor never care about your background, race, age.