Sorry to say, but he may only have been in the relationship for the sex.
Maybe he stayed with you until you were ready to have sex, and when it happened, he got what he wanted, so now he wants to leave you.
But considering your age, I would expect both of you to be more mature, not exactly kids in high school, right?
But I can't really say anything. Your boyfriend could be a great guy at heart, and there may be another reason that you don't know about that better explains his behavior.
I don't personally know you, and I'm making assumptions here, so please correct me if I'm wrong.