What is the ruling on dealing with a non-Muslim doctor who is feared to be a deviant, despite his good treatment and dedication to his work? Is it permissible to take another course with him?

This man is a foreigner, and he must be treated as such, according to the rulings concerning a woman's interaction with a foreign man. Caution should be further emphasized if he is a pervert. If it is possible for you to study under a woman, then it is not permissible for you to study through a foreign man. However, if you cannot find a woman to teach you this course and you are in need of this study, then there is no objection to him teaching you, provided that the legitimate Islamic controls are observed and that temptation is avoided.