My 13 year old son talks a lot about having kids. He says he wants them asap. What can I do as a parent to tell him to wait for children?

Teen wants kids. I would not be too worried about what a 13 year old says about having kids. There is a lot of growing up that still has to occur ! However, if you want to teach him what is required to be a parent let him see what is required. Let him see the sacrifices necessary to be the parent of a child. He may slow down the clock when he sees his needs come second.
Having kids. Is your son an only child? Does he have friends? His wanting to have kids may just be an expression of filling in a void in his emotional and psychological being. Doesn't he get enough bonding from you and your espouse? Since he is too young to grasp the realities and responsibilities of his obsession, he needds psychological help. Please bring him to a psychologist or psychiatrist.
Financial planning. I hope you are taking every life event to help train this kid about finances, cost, work and saving. Does he do chores ? Get an allowance?Save up for his expenditures? You really don't want to paint a picture of kids as a burden, but he needs to understand the actual investment in time and money that come with the territory. He is at a good stage to contemplate these things,but relax, he'll learn.