I want another baby but partner dosen't. Why is it such a big deal?

I have been with my boyfriend for 11 years and we have a 10 year old son. He was a very difficult and clingy baby that didn't sleep well at night. It really tired us out and made us have arguments so my partner and I decided he would go get a vasectomy. This was done when our son was 8 months. Two years ago I felt really broody and longed for another child. This went on for 6 months before I talked it through with my partner. He wasn't very keen on getting a reversal and trying for another child. He said he would do it but I felt like I was forcing him so it didn't happen. Since then I still feel really broody and desperately want another child. I'm 36 now and don't want to wait too long but now my partner is saying it's too late now and our son is growing up, why go backwards? I can sorta understand this but it I still want another child. The feeling is really strong and financially we are fine to afford one and I would stop at home in it's first years of life. This is causing resentment and arguments now. How can we resolve this? I don't want to leave him at all but I don't see what option I have.