what to do......

Lece
My boyfriend and I have been together a year, we have known one another for about 5 years. He's like my best friend.. he has three kids (two not biologically his) and I have 1 child (from a past relationship. He's 28, and I am 22... I want one more kid, he doesn't. 
Now, I know together we have 4 kids but I still can't stop my want for one more kid. He now says he is "willing to think about it" but if the subject comes up he turns it in to "well YOU are the one who wants another kid and you knew I didn't".... after seeing everyone around me (4+ people) become pregnant, it just discourages me... I feel like the relationship won't work if we can't agree one something that big... which is also discouraging..
We agree on most everything else.. we get along great. He's good to my son... he's a good man.. I just feel lost.. any help would be nice, and please, nonnegative comments!