I realise that I had not really adressed your question- only one response to it...
Yeah, what happens when you have a kid who knows more about someone's body- and how it came to be that way than the person himself?... it sure puts you in a pickle.
There are so many parents who seemingly try to intimidate others into circumcising with, "Aren't you afraid he will be made fun of?" (Implication: teasing a intact child is justifiable, so much so that I should turn my intact child into a circumcised one in order to avoid it)
and I always *want* to come back with, "Aren't YOU AFRAID my son is going to tell him what you did to him!? After so much as telling me that you are going to teach your son to scorn my child's body- you expect me to instill charitable compassionate character in my son to protect YOU?"
Most intact kids are so horrified by the idea of having a piece of their penis cut off that they are immediatly filled with compassion and kindness towards a circumcised peer. There is no "INTACT CLUB" mentality the way the "CIRCUMCISED CLUB" defensivly exists to prop up a reason to feel good about having lost a part of your penis.
Just tell your kid the truth and explain that it happened when they were a baby and that their parents believed it was best for them. Depending on the circumstance of the issue coming up, it may be a matter of simply explaining the difference. It is almost never a circumstance of cruelty- usually the question is a genuine curiosity between two friends who like eachother.
"When you were born yours was just like mine, my older brother is circumcised like you, but my mom learned that it is not necissary to do, so when I was born they left mine alone. "
The stories I have read from restoring men about the moment when they first found out they were circumcised are sometimes very tragic- this is because the boys are left wide open by their ignorance- you never know where they will be or who they will be with when that anvil falls. I do not think that it is fair for parents to avoid the issue by not talking about it.
I think a lot of parents are afraid to not circumcse the second boy because they can not face that talk... they would rather circumcise two and gamble on how those boys will ever find out- than to be brave and take responsibility for explaining what happened.
I really respect you for your courage to say no and deal with the various issues that may or most likely will not come up. I think you will find that children are amazingly accepting if they are respected. They are a lot more kind to each other than we give them credit for, and there is no flip-side to the circumcised peer group, if there is one, it is only a backlash but it does not have it's own generating force. there are only friends... there is no circumcised and intact vengeful backbiting- this lockeroom teasing is overinflated.
My son had friends who are intact and circumcised- and right now they are all ignorant of one another's "status" so what is going to happen when they do find out- you think friendships are going to crumble? segregated playdays? new tribes? I doubt it.