Just found out my boyfriend might have a son

So tonight my boyfriend says he has something to admit to me. So I said ok what’s going on. Then he tells me a girl that he cheated on with his ex. Got pregnant around the same time they had sex. The girl found out that she was pregnant but told my boyfriend that the kid is not his. That the father is her ex’s. This happened in 2016. But today my boyfriend says he thinks the kid is his. The kid is now 3 years old. Right now I feel like I have no one I can talk to my boyfriend doesn’t really want to talk about it with me right now. And I feel like he’s keeping other things from me too but I’m trying to stay positive and supportive. I knew he cheated on his ex but I had no idea about the kid.