I have a few questions.
Does the school have an atmosphere that has zero tolerance for bullying from student to student, student to teacher, teacher to student?
Is your child's giving nature the "norm" amongst students and faculty at the school?
Is respect, tolerance, sharing, and a sense of community taught as part of the daily routine at the school?
If not, your child may be going against the grain and probably ruffling feathers. S/he is, in fact, probably making the teacher look bad. I know this may sound weird but some teachers are control freaks and if the child is being good it renders them powerless. It's like when you kill someone with kindness, but this is unintentional. And so the teacher projects these feeling of insecurity onto the child in the form of negative comments, jokes, or condescencion. I would make an appointment. But be smooth about it. In this meeting, give a little background about yourself, about the child's previous school, and ask about the child's academic progress, and your expectation from your child and the school.
In the 5th grade, my daughter was one of the smartest kids in her class, but she worked slower than the other children. The teacher tapped on my daughter's forehead with her pointed finger and told her she will always be slow, embarrassing my daughter, as she forced back the tears. When my daughter came home and told me, I was infuriated. I made an appointment to meet with my daughter's teacher, discussing her grades, her time management and the incident that my daughter reported. I was very respectful and cordial while being strong about my concerns as to how my daughter is treated in class. If the teacher treats the child bad, the rest of the class may follow suit. After that meeting, I continued to keep open communication with the teacher. I never heard anything else negative after that. Unfortunately, many teachers forget that, many student come home and tell their parents everything. My daughter is now a sophomore. She is in all honors classes. She still works a little slower than a lot of the kids, but we consistently communicate with the teachers. It shows them that you are a concerned parent who has a vested interest in your child's school life. Good luck and count to ten before you say anything after the teacher may say something you don't agree with. If the problem persists or if you feel that the teacher may feel that you or your son may be overreacting, go to the principal.

That would be my first course of action by meeting with the teacher and the student and having an honest conversation about the situation and express your concerns.

If that meeting proves to be unproductive, I would then make your concerns to the principal and if that doesn't work, then have the student placed in another class.

That is disturbing, though, and a bit unprofessional on the teacher's part.