Anyone have any good books that will help me find the words to start that conversation with my son?

My son is 4 1/2 and I’ve recently learned his first mom is now starting a family. I haven’t heard from her directly in quite some time. In other words, if she were present in his life this conversation would happen more organically because he would be seeing the changes in her life firsthand.

Anyone have any good book titles aimed at the kindergarten set that will help me find the words to start that conversation with him?