I'm sorry to hear about your situation, it sounds really tough. Well, I guess it's really hard to explain and it probably won't sink in or he wouldn't really understand it fully until later in life. So, maybe it's just best to make the transition smoother, rather than forcing yourselves to explain fully or forcing him to understand. By this, I mean probably an explanation from his mother, and constant answering of questions. The best thing you can do right now is to just be open to questions and to get ready for a long time of explaining. I'm sure he will be confused a lot more times in life about this beyond this initial stage. Just make sure that he knows you are open for discussion no matter how old he gets, and that he knows you will be explaining it as best you can along the way. Hope this helps and good luck!