He's 16 next week and has always said he would not have sex with his 17 year old GF until it was legal. He has asked for her to sleep over or him to sleep @ hers next week. We are very uncomfortable with allowing this as although we've talked about precautions/bought condoms etc. we think the less times they have sex the less chance there is of pregnancy. We know we can't stop it from happening at all. It's a numbers thing. We know he will not be happy when we tell him. Any advice on how we do?