the kid is obviously not worthy, so i just say bring him to the nearest dumpster, toss it into a bag and well, you get the idea
my sister is adopted, and my parents have always been honest with her about it. she is the oldest, and my brothers never knew until they were older because they just didn't question it. she looks like all the rest of us, though, so no one really questions it. sometimes people say, "you guys look so much alike! and we just smile at each other. i would say they should just be honest with him and explain things as best they can. if he doesn't understand right now, he will someday.
Tell them the truth as soon as possible. It might feel like their life is a lie telling them later. If you tell them sooner, they won't judge. Parents are people who raise kids, not people who have proved they can get knocked up and carried to turn.
My dad was adopted and he's always known and never made a big deal about it... I'd say just be honest with your kids!
explain that mom couldnt get pregnant like most moms do, so she went and chose the kid instead, blah blah blah. still the kids parents, just didnt give birth to the kid, etc