You seem to not be listening to everyone who is saying the same thing - you’ve rushed into this.
Marrying someone within a year of meeting would be ridiculously quick for anyone, more so anyone with kids, let alone kids with issues.
Having a baby was only ever going to inflame the situation with your son. He has no/little relationship with his dad and now he’s being pushed out of his relationship with his step dad by a new baby (whether true or not that will be how he sees it). Of course his behaviour will suffer.
In relation to your DH, he’s nearly 50. That is old to be having to start again with babies while dealing with another 3 kids (that aren’t his) nearing the awful teen years, one of whom has worsening behaviour issues. What was the logic behind another baby when you already have 3 and he has grown up kids. What would another baby bring to your situation when you look at the other factors (ie how it will negatively impact the house).
Bluntly - he’s old, stressed, knackered and it sounds like he’s now depressed and struggling to cope. And I’m not sure I could blame him. And you can talk about the ideal picture of a blended family all you like, but your kids are not his and it’s hard enough to deal with such issues when they are your own, let alone when they’re not.
However, to a degree all this is irrelevant. You are where you are and you BOTH made your choices.
You do need to understand the enormity of how much his life has changed and support him in he same way he needs to support you. I don’t think you want to understand / acknowledge just how much he’s taken on so quickly with little adjustment period, perhaps because maybe you’d have to acknowledge some mistakes were made. But you can’t move forwards if you don’t.
He needs to find strategies to cope (as do you) and you need to talk. Call him, ask him when he’s coming home and go from there. Discuss what you both need, even if it is time off. Sometimes it’s easier to deal with something if you know you can escape it soon for a bit.
And if possible you need to let this episode go - maybe explain you feel unsupported and don’t want it to happen again but don’t hold it against him (particularly if his mental health is struggling) if he works to support you going forward. People are allowed mistakes.
That sounds a bit like I’m having a go at you and I’m not - but you need to face facts to move forwards. I wish you luck.