I agree with you, Ox. You must follow your instincts or will suffer consequences later (he will too!)
Not that I am all-knowing, but when I find a question from a poster that I can identify with or have been through the subject, I feel compelled to reach out and help (hopefully).
IMHO 20 years old is much to young too get married.
Please talk to him asap and tell him exactly what you said here. Enjoy the courtship, and, if he truely loves you, he will wait for you. Set some goals for yourselves: finish college, get the job, travel, build a sweet nest egg, and HAVE FUN. Then, revisit in a few years. Nothing wrong with a year+ engagement!
Good luck and best wishes