I'm sorry your son was embarrassed by a teacher. I would suggest contacting that teacher. Explain what your son told you and how it effected him. Then listen to what the teacher has to say. The story might be different than the one your son told you. Keep an open mind and be respectful. I'm sure if the teacher said something that was considered embarrassing, he didn't mean it to be hurtful. A good teacher will take what you have said and try to correct the problem.
If the teacher continues to give your son a difficult time, report it to the administration. Go in armed with dates of the occurrences, actions of the teacher, and reactions of your son. Stick to the facts - it will make for a stronger case.
If your son's eating habits don't fit the norm, then show him a more acceptable way. For example, my nephew used to eat with his mouth open when he was younger. After spending two weeks with me, his habit changed.
Also, you can empower your son by giving him things to say to the teacher if it happens again. For example, I don't think that was a nice thing to say, or please don't embarrass me. Standing up for yourself is a life skill.