Worried on Relationship

G is really - how should I put this? - tied up to me!
Last night, he mentioned the most horrifying thing to me; more or less, about marriage. I silently freaked myself out (and not in a good way, either). I'm 15, currently dating but with no dating experience. My guy knows it but doesn't take this relationship step by step, instead of doing just that he went over a few steps which normal couples would think about after 2 to 3 years of dating. However, it only took him 2 to 3 nights! I know that he's 17, being that now is his last year in school and he's gonna get a career after graduation and will be needing a girl in his life but seriously, I can't be that girl!! I am totally freaking out and couldn't breathe at some point.
I love it that he always says 'I love you' and 'I miss you' all the time but please lah, I need some space! I don't need you to text me from morning to night. I'm starting to get annoyed with this but I still pity him. I really do love him but I can't make any promises and neither should he. I want the both of us to just enjoy a non-married life after graduation. I don't want to be home, sittin' with the baby, envying my single friends at the clubs and waiting on my guy late at night when I'm 21. No, I wanna be wild and free and do whatever a non-married 21 year old would do.
I haven't told him this but I will tomorrow. I guess I'm also tellin' him I don't wanna get married at all. Let's see how things go from here...