If he has two moms, he can say so. If they want to know more about it, they can ask you. Just as you do not have to answer every question posed to you, your child also has the right not to answer every question he is asked. He has a right to his privacy. Tell your children, flat out, that they do not have to talk about anything that makes them feel uncomfortable.
Different children draw the line at different points.
Paula and Nancy's son Thomas recently did a school project where he made a family tree, complete with the man who was his sperm donor. Not all children wish to be as open as Thomas. Starting at the beginning of the elementary school years, you should talk to your child about the issue of privacy. Let him know that sometimes people ask rude questions, even if they don't mean to be rude. Many lesbian mothers report that their house is the one where the neighborhood children prefer to play.