I don't think the word "let" is the best way of asking this question. As a parent myself (although mine is quite young..and being in your position is a thought I don't want to think about right now..)
If your boy is legally of age.. (i.e. 18 in the US..) then there is nothing you can do to stop him from sleeping with his girl even if he lives in your house. We set rules for our children and we try to guide them along the ways that we have deemed acceptable but at some point their decisions are their own.
My suggestion is..pull up some videos on YouTube of labor and delivery. Go to babycenter.com and look up in the tools the baby cost calculator. If that doesn't scare the horniness right out of him, nothing will.
If you are talking about "letting" him sleep with his girl I assume you've already taught him a number of things about sex and how to prevent disease and pregnancy. You can only hope that the things you have taught him are instilled in him.