I think it *does* matter, but I think you answered your own question: you and your DS need specific criteria so that you understand what is expected.
Different teachers are different, of course, so take this with a grain of salt, but when I was teaching, my main criteria for giving good scores for class participation was that a student contribute positively to the class in some way. That means that the dingbat who talks constantly but says nothing gets a worse grade than the person who makes one brief, insightful comment or who has good things to say during some small group work. Some people do great in public and others prefer to contribute in other ways. I would encourage the teacher to recognize this fact of life in some gentle, not-pushy way.
FWIW...(not much!)
