ANDY'S ANSWER: If you've never dicsussed sex with your teen before, it's probably not going to fit into a casual conversation under any circumstances I can think of.
I think the best thing to do is address the current situation head on and ask your son what his thoughts on the matter are. What you are going to try an accomplish is him considering all not only the exciting cool parts of having sex, but also what the possible consequences are. If he decides to have sex, how is he going to protect himself against a sexually transmitted infection? What would it mean to him if the girl got pregnant? How would that affect his life? How would he prevent that from happening?
Any one wanting to start a sexual relationship who is not willing and prepared to take responsibility for birth control is not rmature enough to be having one. I mean that both partners should be taking responsibility, not just one.
I know so many young men who have become fathers at a young age because "SHE was supposed to be taking the pill." Poppycock, I say! If you are not ready to become a parent of a living human being who is going to need you for the next 18-20 years, then take care of the birth control from your end regardless if your partner is looking after it on their end.
Can't afford birth control? I guess that means you also can't afford to take the chance of getting pregnant, can you?
I personally think that sex is a very risky addition to the life of someone who is still going through puberty. For one thing, there are emotional after-effects, particularly once a relationship has broken off which can be very traumatic for teens who are already struggling with the stresses of trying to define themselves as a person.
However, I also know that as parents the very best you can do is provide your children with the tools to make good choices for themselves, because all the most important decisions they make will be made when you aren't there.