I am desperate for another child whilst he cannot see past the additional expenses this will incur
Having a child is something both parties need to be on board for. It's not just expense (although that is huge - the money you spend raising a third child would be a deposit for a house each for the other two). There are a lot of other factors to consider.
I expect you're picturing a healthy, able bodied, neuro typical baby who slots into the family and everyone dotes on. Have you considered the impact if your third child has additional needs, is severely autistic, has a life limiting condition, mental health issues, etc? If you already have two healthy, typical children, you might not want to not roll the dice again.
Basically, your choices are :
- stay with your husband and accept that means your family is complete
- leave your husband and have a baby either by yourself or with a new partner.
I don't think there is a secret third option where you magically persuade your husband to want a third child. I suppose you might be able to beg, manipulate, threaten and punish your husband into giving in, but is that what you want your family to be like?