I'm probably going to get ripped apart for this but I kind of understand him. I'm a step mum and while I love my step son when my daughter was coming into the world with my DH I was extremely vulnerable and felt unimportant because I knew DH had shared this special moment before with his ex. Every single living being on this planet has the ability to feel vulnerable, or left out or unimportant (not that it's anything you have done).
You might find the reason he is lashing out/acting aggressively etc is because he feels like everything is spiraling out of his control and may not feel like an important role in this process of his first child being brought into the world.
Not wanting to offend anyone with this post but being a step parent myself I do understand the challenges and emotions you can sometimes face when your self confidence is at a low.