Hmmm, sounds like you could tell him, something like, "I just want you to know that the four of us are friends, and it's like as if we were married with them. If you have any questions about that, I'll be happy to answer." He sounds like a smart kid. That way at least you won't feel this burden of secrecy, and kids have a way of guessing at things, anyway.
Sorry busbuddha hasn't had as good of luck in coming out,