I've been dating almost a year & we are expecting. we started our relationship as a mono one (with zero connections to polyamory in anyway), but some weeks ago I stumbled across something he wrote & apparently he has 'realized' that he is poly, but the post was completely negative about our current relationship. This took me completely by surprise & I've tried to talk to him about it, but every time I ask him anything he tells me I don't know. I'm hurt & confused & getting zero answers.
He could be feeling angry or defensive about the way you "stumbled across" what he wrote - was there a violation of privacy on your part? You may need to take ownership of that and apologize to him, and try and re-lay a foundation to have that conversation safely. "I'm sorry that I found out this way, and that I found out before you were ready to talk about this with me - but this is where we are now, and I can't pretend to un-know what I know."
Ask him to make time to sit down with you and talk things out, straight and clear. Ask him direct questions, like "would you like our relationship to change? In what ways?" or "what about this relationship is making you unhappy?" or "what do you need from me to feel safe talking about this?"
If he refuses to answer your questions, you're allowed to press a bit more: "Look, we are about to have a child together, so we really need to get this figured out. I need you to be clear, open, and honest with me. I can't promise that I won't be upset or hurt if you share difficult things, but I can promise to receive what you have to say and give you a safe space to share how you feel. You need you give us the chance to figure this out together."
If he simply refuses to engage with you on this, then you have a choice to make: do you want to stay with someone who cannot or will not discuss his feelings and needs with you?