Man this thread is like a beacon shining on the double standard these boards are full of every day.
"I'm a man, and I have two kids with another woman. I wanted two kids with my current DP. She didn't. I threatened to leave, and she agreed to have children with me. After one child, she said she doesn't want any more. What can I do to convince this woman to have another one of my children, because I want two kids."
?
How many "Oh, that's such a reasonable position" replies do you think we'd get on a thread like that?
How many "Fuck you, you're a man, you don't have any rights over a woman's body" replies do you think would get posted?
Do you think we'd get the suggestion of "You should get another, different woman pregnant"?
I think not.
His body. His decision.
If you hold the opinion of "Her body. Her decision." then anything less than that for men is just rank hypocrisy. It's pretty much the definition of Equality.
OP: There's nothing wrong with only children. Your DC is going to have a couple of siblings already when they grow up, even if there's a bit of an age gap and they're step- rather than blooded siblings. It doesn't mean they can't have a meaningful relationship with your child.
If it's a huge problem, and is going to trash the marriage, then you need to leave him.
If you're going to resent him forever, you need to leave him.
If you believe that your wish to have 2 children is more important than his wish to not have 4 children, you need to leave him.
Otherwise you need to accept it and move on. Because not leaving him and not forgiving him together is going to slowly poison your relationship for the rest of your lives.