I’ve been in computer science a year and a half and I’m switching out after this semester. I’ve had the same professor all that time. Even though we’ve always had large classes, he knows who I am because I used to go to his office every week. I’m very thankful that he helped me even when I doubted myself and he always was super nice to me, but is it weird now to send an email and say “Hi professor, I wanted to let you know that I’ve withdrawn from your class because I’ve been unhappy for a while regarding CS, but thank you for helping me with CS these last couple semesters and being an excellent profesor” or is that totally strange?
I don’t believe it’s a bad thing to express your gratitude. I’m sure he would be happy to know he has made an impact on you and your academic career. Being thankful in this day and age is something our society can use more of. 
Always nice to say thank you.
It is fine to let him/her know that even though CS didn’t turn out to be the right major for you and you withdrew from the class that you appreciate all of his/her help while you were in the classes.
Very classy!