Ex will have another child, doesn't want our kid to know

Sorry for any mistakes but English is not my main language, please bear with my long story.I'm a BM to a boy who will be three in January. His father and I broke up 25 days postpartum because of his cheating and gaslighting. We remained friendly and have a good co-parenting relationship. I have a sole custody and his father sees him at my place once a week for a couple of hours. We are all happy with that agreement. I'm happy because I can't trust my ex with our son, ex is happy because he doesn't like obligations and son is happy because he likes to play and spend time with us together. Shortly before my sons first birthday ex started seeing new girl, his ex. We never talk about our private lifes but I heard it from mutual friends. They allegedly have a very turbulent relationship, he sometimes hits her and cheats, he even told me couple of times that we should have one more child (Yeah, NO, I only bear with him for the sake of our son). She never met my son, nor does my son know that his father has another life.Now the problem, this week my ex called me and said that he will have another child. He doesn't know when but thinks sometime next summer, he doesn't have a plan. He told me that he doesn't want our kid to know about it, that us is us and they are they, that nothing will change with our agreement. I told him that we can't hide it from our kid, it's not fair, and we live in a fairly small city, he could found out about it in day care or school. Ex told me that we will see what happens in 20 years, WTF??!! I don't even know is this girl that I know about pregnant or somebody else.My question is what is the best aproach with this to my kid? Do I tell him? How? When? What to do with my ex and our agreement? Does somebody else have a similar situation? via /r/Parenting https://ift.tt/2Jfyk7K