I would tell him that I plan to have another baby with donor sperm, I'll be taking maternity leave, and am staying put in this house with my first child to raise that baby
I agree. This is what I'd do, to be honest. This is a man who's always known you want children, as in more than 1 child. He's holding your fertility hostage as he's living the life he likes. Your DC is 8 - why hasn't he got a part-time job, at least? He doesn't want to be a SAHD to another child, according to him. So he's ok if it's 1 child then, as I guess that's a whole lot easier.
As difficult as it is, there are 2 parent families out there with both parents working, and the children in nursery. & you'd get maternity leave wouldn't you?
On the other hand you could equally have known he never wanted children, and hoped he would change after you had a child and then got married.
I think there's a bit of wrong on both sides here. However - you want another baby. It's your right to go for that, just as it's his right not to want a child. In which case you have a child but not with him, and take the risk that your marriage will disintegrate.
You can't have it both ways