My Boyfriend Doesn't Want To Have A Baby W/ Me.?

My boyfriend already has a baby with his baby momma. It was an unplanned pregnancy and he told me that he did't want the child then and still doesn't. Now his and my relationship have progressed to 4yrs now and I'm ready for a baby. He tells me he wants one with me, but every now and then he'll change his mind. Whys do you think he doesn't want one with me????