we have dealt with a lot in the past few years (my mum had cancer, we brought a house of horrors that took much longer to renovate, covid) and now things have settled down I thought we would be thinking about the next steps in life together but it seems this is it.
If I'd gone through all that upheaval and turmoil and come out the other side in one piece I'd want to take some time to enjoy what I had rather than taking "next steps" which will be all-consuming and involve a lot of change.
Your DP's reasons for not wanting to get married or have another child are perfectly reasonable and he's been honest with you.
You could still make your argument for those things, they're conversations worth having. He should hear you out and have a respectful discussion but obviously he's not obliged to change his mind. You have a stronger argument for getting married, and you should focus on wanting to do it for your own legal protection. Meaning the world to your family is neither here nor there and not something that would sway me - the social pressure to get married because it's the "normal thing" is precisely why many people don't want to do it.
However like PP I suspect that the root of this isn't marriage or even another child but your lack of love for your DP. You don't say much about him as a person, which is telling in itself, but I don't get the impression you're satisfied in the relationship.
Given that there doesn't seem to be any abuse, you owe it to your son to get to the root of that before leaving. Did your DP change, become boring, become dull? What made you fall for him in the first place and can he get that back?
Or did you ever fall for him in the first place? Did you love him for who he was or was he only just "fine" and "OK"? I'm afraid if this was the case it won't get any better, but it's also a case of lying in the bed you made for yourself.