I'm so sorry you are having to put up with those kinds of comments. Unbelievable.
Your son is very fortunate to have you as a mom - someone who was willing to stand up for his own right to decide what happens to his own body. Someone who was unwilling to allow others to pressure her into hurting her son unnecessarily, and taking away an important part of his sexuality. You're doing a great job!

As your son gets older, I would make sure he understands what circumcision is and what it takes away from a man. I would make sure he understands what his foreskin is, how it functions and how it is the most sensitive part of a man's penis. Obviously, tailoring the information to you son's age at the time!
But if your son is educated about the foreskin and circumcision, he will, I have no doubt, be grateful that you protected him.
As for the people making negative comments - I would also make it clear to them that you are unwilling to continue to listen to these comments, and that under NO circumstances are they to say anything like that to your son.
How would you feel about finding a good article about circumcision and printing it out for some of these people? Then you could tell them that once they'd read the article and are fully informed about how damaging circumcision is, THEN you'd be willing to discuss the issue with them, but not beforehand.