A: Dear talk2much841:
This is a situation that is bound to be a bit confusing for your son, so I would encourage you to support your son's efforts to understand it by answering his questions and providing brief explanations that are "child sized" and as reassuring as possible. Your son may not have a very clear idea why you and your son's father don't form a family to start with. Why his father is now saying that he has family with his new girlfriend and her little boy is an additional mystery.
I would take things as they come. In a way, your son's father is trying to express that he loves both your son and the other little fellow, and that they ought to be attached as brothers. In this way, the father is expressing that your son has not "lost" his father, and that your son continues to be a part of his father's life no matter what. This is a good message, because it emphasizes the love relationship.
If in the future your son's father breaks up with his girlfriend, it is possible that the other little boy will disappear from your son's life just as the girlfriend will disappear. Of course, in that case you would want to explain these events as best you can--but for now, I would act as they are all permanently attached until you learn otherwise .
Perhaps you and your son's father can have a friendly chat about how best to communicate with your son around this issue and about the many questions which will come up as he grows. It will help your son a great deal if you and his father can have a reasonably constructive ongoing interaction. That you and his father have a friendly relationship is probably much more important for your son than any specific thing that is discussed
Elizabeth Berger MD
Child Psychiatrist and author of "Raising Kids with Character"