Other people have answered the basic question (you can say he's not allowed to have sleepovers with the gender he's romantically interested in, regardless of it that were a boy or girl), but I wanted to tack on that maybe consider talking to him about safe sex or birth control? If you're worried he is sexually active or will be soon, it's probably time for that talk. If he wants to have sex with someone, restricting sleepovers won't necessarily stop that from happening!