You just need to talk to him and explain all these things you are worried about. You might need to be prepared that the fact you have described your relationship as ‘not great’, admitting this as a fear to him might make things worse. Perhaps look into some sort of couples therapy as the situation can’t be helping your anxiety or stress levels anyway.
I think you e kind of answered your own question here in a way I’m so sorry to hear you’re both not in a good place 😢. Maybe try and speak to him and concentrate on this first? I have a 3 year old and 10 month old it’s tough I couldn’t imagine doing it with struggles at home too (however I’m sure some do it with struggles) but wanting a sibling shouldn’t be the only reason really it’s hardwork! At the end of the day you have both got to want it and I’m sure he can just go to work and very day? So ultimately it’s on you? It’s really hard have you both always said you want more than one? Xx