I personally think that honesty is always the best policy. Always be clear with your needs, feelings or apprehensions. If you're unsure about having more kids you should have a conversation with him and discuss it openly and honestly with him. Maybe then, you be able to see more clearly (1) why you don't want more childrens , (2) is the fact that he does want children a deal-breaker for you relationship (something you can't make a compromise on), (3) what can you as a couple and a person to work through what you both think and feel and then (4) find common ground to look at solving this and being open about what you both needs. Marriage is not something you should be entering with doubts and uncertainty...If you plan to marry that man, be honest with him about what you want out of your life together; more childrens or not.

I have a friend who was that 'man" who wanted children...his girlfriend knew deep down she didn't want any because she didn't have the capacity for it as a person (that's her opinion)...but she leads him to believe that eventually she would makes him a proud father...so she wouldn't lose her boyfriend. I thinks that a selfish way to do things especially in a couple and it then come to affect the trust in the relationship when the truth gets out.(which always do)

In a sense if that's a deal-breakers for him to have childrens...Loving him apply also to not forcing him to give up something that matter deeply to him when you know you can't honestly give it to him...If it comes to that, which i really hope for you it don't (first talk to him before worrying about the outcome), then it's not meant to be...Being with someone implies that you have to respect yourself by not doing things you know you don't want to do (like having a child because he wants one) and also respecting the core needs of the one you love (like the need he might have to have more childrens).

I wish you good luck...and try to have that talk and let's me know how it when!:P Take care.