He may just want to tell you what he thinks you want to hear so he can get in your panties. You could play some silly head games and tell him you don't believe in marriage and see what he does. If he loses interest in you, you know you blew it, but at your age it doesn't matter.
People change a LOT before they are 25. They change their goals. They develop their habits and set their course in life. Making commitments at your age will most likely be a total waste of time and effort. Concentrate on you and your future and you will find someone right. If he's the right guy, he may even wait for you. Until you're older, you're the most important person in your life. If your life is on track and you have your sh*t together, you will be much more of a marriage catch for a guy.