When you speak to him, encourage him to share his fears/anxieties about the situation. What does he believe is going to be different, does he know of anyone of his peers/friends who is facing a similar issue and what is his biggest concern. It appears that you've already started preparing him for this new situation by explaining to him that sometimes people split up for various reasons that they're dealing with. It has nothing to do with him (younger children usually believe that they may have caused the split) You'd want to reassure him that you and his mom would be there for him and just because people do not live together with him it does not mean that they do not love him. He will most likely experience loss and go through the stages of grief not only of the person who's leaving the relationship but also of their routines together. At times you'd want to be there for him so he can just talk about it if he wants to. Depending on what tone his mom is projecting when she talks about her partner, you would want to remain neutral as you do not want him to become consumed by angry or resentful feelings for too long. It is natural to experience these but they can become harmful if he ruminates and becomes drawned by these.