Also... how do you know that your son has only a vague idea? Kids are much more clever than their parents tend to give them credit for. The only way you could "know" that he doesn't know is by asking him, which is impossible to do without spilling the beans. How can you be sure no one else knows, and that he hasn't heard it through the grapevine. Would you rather your son learn about your relationships from his parents, or from the kids at school teasing him about it?
I know he doesn't know because I asked him if he thought his father kissed "X" his father's girlfriend, and other such questions. Yes, it is possible without spilling the beans. So he wither doesn't know or doesn't want to know. How would any kids at his school know about my sex life? LOL