Hi C. -
I would absolutely let your son's teacher know about all of this! She may have noticed already and she may have some observations to share with you, or she could be unaware of it and it would be a good thing for her to know. Having brought it to her attention, you can ask that she keep her eyes and ears open for such interactions. That said, not all teachers are very effective with stuff like this -- I don't know if it's a lack of training, a lack of time in a busy day, or a lack of understanding (I won't say a lack of caring). So you'll need to do some back-up support as well. We've been there with our daughter and so have some of our friends with their kids.
There are some excellent books on this subject, one that comes right to mind is "The Bully, the Bullied, and the Bystander". I am NOT saying I think of what you describe as bullying. I saw one of the mom's comments that it could be that the other kids aren't communicating very clearly and I think she may be right about this situation. That certainly is true of children that age.
Something that goes on in groups of kids (and adults), not necessarily consciously, is that they try to establish a social order. By identifying who is the smallest & youngest (and by extension the weakest), they can identify themselves as bigger, older, stronger. And then there's the range in between. Not everyone has the need to do that, and it's bewildering to those of us who are content to just be ourselves.
Actually, I was there myself and grew up feeling a bit bad about myself. I came around to appreciate my sensitivity and differentness and even celebrated my ability to be empathetic. But all that was hard-won and took a long time. I want very much to point out that I was younger than all of my classmates. And I urge you, if you're not already considering it, to let your son have another year of pre-K before moving up. There is no shame in it; it's done frequently now and kids really benefit from it. I spent 3 years wishing I'd done the same for my daughter but I didn't realize her non-readiness (in her case academically) for Kindergarten.
Sorry that got long -- as I went along, the personal aspect of it sort of came up but it helped me to be clear (I hope). This is important stuff -- the readiness to move up to the next level at the right time. Learning how to navigate the negative comments of classmates is important too. I loved the suggestion by the mom who said to ask your son what HE thought and to help him see that was the main thing.
I wish you and your family all the best,
Colleen