Be aware that at 14, he is more likely to be smoking dope than having sex.
As far as sleepovers go, the issue is supervision not girls. In a group of 14/15 yos, there will be some who push the limits. At an unsupervised sleepover in a house with no adults, there will be some kids drinking, taking drugs or having sex - absolutely definitely without any doubt - because those who like to push the limits are drawn to places where they can. But if you host a sleepover that is 'publicised' as being in your front room with you around, then there probably won't be, because those looking for a place to push the limits will know your house isn't it.
Communication is key. With your DS's friends as well as with him. IME, the friends who have looked you in the eye, had a conversation with you and/or eaten a meal with you are v unlikely to do things in your house that they know you would not like.
Make sure they all know the rules. Don't leave it to your DS to tell them what's allowed, because he won't want to. I have one friend, a former youth worker, who has been particularly good at saying clearly, breezily and in a firm-but-friendly way, things like "Hi, welcome! Come on in! You know this house is sex and drugs free don't you, until you're all legal?!" There's always a bit of teenage blushing, but her house has been really popular with teens. They do like to know where they stand. :)
This age is especially scary for parents, because you start to lose control of them, before they feel old enough to be responsible for themselves. I'd say the main task ahead of you is to support your DS to make good decisions, because he needs to learn how to control his own behaviour, now he's too old for you to control it for him.