I think this is a 'let it go' scenario, because there's not much you're going to be able to say that will get someone to see it your way. If you bring up all the worst case scenarios that could happen, you'll just upset yourself and make the other person think you're being dramatic. I know I get impatient with one of my coworkers who, from my perspective, seems to focus on the one in a million bad things that could happen to her kid. She tries to drive the point home by telling me scary child abduction stories, but the statistics still say it's really rare.
So, if it bothers you, I say just change the subject. Many of them are probably not trying to be dismissive, especially when you yourself are saying he's doing fine and it's 'just' a few spots and so on. It sounds like you have a good group of other parents who you can really vent to, so do that with them. Make a sympathetic-only group for facebook posts and so on.