Dad not as excited this time around?

My SO has a child from high school with someone he actually hates. Every interaction is tortured and I nothing about the situation is healthy. We stay as far away as possible and stick strictly to the agreement.

Now I am pregnant and we heard the heartbeat for the first time yesterday. He told me all about how he cried with his first child when he heard it. This time... nothing. I’m sure it’s nothing and I should just forget it but it’s difficult. I wish he would keep info about the first to himself if he’s going to be so underwhelmed about this one. Probably just hormones making me crazy but I can’t shake that this is just old news to him.