[spaypets]

Is your son's last name different from your husband's? I'd suggest using that as a starting off point. And tell him now, rather than waiting until adolescence.

Here's a story you might find instructive. A childhood friend of mine never knew that his "Daddy" wasn't his bio father. I suppose his mother never intended to tell him. But when he was about 16 he started dating a girl--a girl who it turned out was his half sister (same father) -- naturally his parents had to tell him then--it was pretty upsetting at that point (and sort of icky to know that he'd been dating his sister).

The story ends sadly--he was killed in a stupid accident when he was 18.