I just need some advice...

I have a two year old son with my ex fiancé and my boyfriend has two kids from his ex wife, 3 and 1 1/2. He and I have known each other since last year but we've been together since January and he is adopting my son because he calls him Daddy and is the only father he has known. We've talked about marriage and living together as a family but the one thing he never touches on is if we will have a baby and I am at a point where I am depressed because I've had baby fever since before we met. I am so depressed that I don't even want to have sex anymore, knowing I don't get straight answers from him. We've touched on it but I don't think he knows what he wants from me and since I live with him and all our kids it hurts more every day to look at them and know I didn't carry them, that they aren't my children biologically. How do I explain to him that this is the reson I've withdrawn for the last couple months?