How do I tell my ob I don't want to have another baby ?

My fiance and I have talked he doesn't want another one this is his 3rd I'm due in April

After this baby I dont want to have anymore

This baby I love to death but I dont want another one I cant do all these health issues again I cant handle my back pain being this bad granted ive been dealing with the pain for 8 years its just getting worse the further along I am

I want my tubes tied (or a different permanent birth control )