Op I think you're getting some harsh responses on here. Realistically how many of us knew our partners parenting styles before we actually had to co parent? Even with (especially older) step kids you're really only getting a snap shot compared to what it's like parenting together full time with a shared child. So there's always a learning curve there. The question for me is if your partner is willing to learn and from your posts if you're willing to embrace him learning.
I think there's a few things here that are jumping out to me,
Firstly I fully understand why your partner has felt the need to jump to help out his son. He's his parent, it's his responsibility to do that. I do think he should have had a discussion with you about it, but I also think when you get with someone who has children you have to accept that there may come a day where those children need to live with you full time and it may not be at the most convenient moment for you both. But you'd need to accept it as a responsibility that your partner has, just as he'd need to accept it were the roles reversed.
The second thing that jumped out at me was that he expects you to be the one to parent and support his son? That to me suggests he's either very lazy (which doesn't necessarily fit with being happy with his kid coming to live with you all with the issues he's likely to bring with him) or he's actually really not confident in his own parenting abilities. It sounds like something he needs to work on so he can step up and be the parent he needs to be outside of you.
The third thing I wondered about is when you say you regularly have to undermine your partner because you feel he spoils your kids and lacks discipline and you disagree with his parenting methods. I do think that can sometimes be needed- but it can also be really unconstructive as well. If you're regularly undermining him, how is he meant to feel confident in his decisions parenting your shared child (if thats the issue) and could lead to him feeling like he needs to constantly default to you and what you think is correct? That doesn't sound healthy to me at all and I would recommend you both go for joint counselling to try and identify why you aren't able to work as a team and how you communicate and support each other. Either that or you ask him to leave and both continue to parent completely separately. At the very least counselling might help you identify if you've got your walls up after previous trauma and need help letting him in a bit more and loosening the reins, or if he needs to step up and own his responsibilities a bit more around the home and not leave it to you to be bad cop.
In terms of the pregnancy, it might be worth going for a few counselling sessions yourself to determine what you want and what the best option is for you going forwards if it's a baby you think you could come round to, or if perhaps you feel like you need a termination. Obviously there's a lot going on all round so having some clear space to really think that through might be helpful and you deserve to feel supported. Plus the talk of you having a mental breakdown etc would worry me in terms of gaslighting and it would be good to have some support thats just your own to process that.