How to get over husband not wanting another child.

I'm so heartbroken. When we first got married we both wanted 2 children close in age. We have a son but he doesn't want another one (he feels one is enough responsibilty). I cry every time I get my period that I'm not pregnant. I know it's not fair to push him but I also feel like it's not fair to me. I never wanted my son to be an only child. I don't know how to cope with this any more.