I guess it depends on how much you know about his past, and previous girlfriends, as to whether you know if he's slept with someone before. If you think he might be lying, he might have his reasons, like not wanting to freak you out. However, don't let him use it to put pressure on you, make sure you wait until it feels right!
And to be honest, if he's just a normal teenage lad, its quite likely that he's just got a brilliant knowledge of porn movies! My boyfriend was 19 when we started going out, I was his first proper girlfriend, and he had always been quite shy. Despite this, he still had loads of ideas and fantasies that he wanted to try out, based on the porn he'd seen! XD