Tough Question from a 4 year old

Although D has mentioned wanting a brother or sister before, earlier today was the first time he really asked why he didn't have one. One of his friends asked him today if he had a brother or sister (his friend has a little brother) and he had to say no, its just me, mommy and daddy. As we are driving home and he is telling about this exchange, he says that big brothers are helpful and he wants to help mommy and daddy make a brother or sister because he really wants one. I had to tactfully explain that big brothers cannot help in that way and that we were seeing a doctor to get help and also hoping that someone picks us as parents. It sucks that he is starting to notice these things more.