22 is so young! If you have any doubts you need to talk to him before he proposes. Imagine having to say "no" as he's on one knee holding a ring! That would be awful and you may get caught up in the moment and say "yes" without being sure...
I also think you should give subtle hints letting him know you aren't ready. Are you in school? If you are, you can casually mention that you want to finish school before getting married so that you can enjoy it and be more financially stable. And when he asks you if you're ready for the next step, don't feel bad asking him straight out what exactly that next step is. Maybe you can also suggest living together first. I know some people don't believ in living together before marriage, but I think its a good idea and it will by you some time!
Good luck! Its kind of refreshing hearing a young girl say she's not ready for marriage... Usually it's totally the opposite way around! (Me being one of the guilty ones). I wanted to get married at 23...Now, almost 3 years later I am pretty happy I didn't have the chance to. I probably wouldn't have chosen to go back to school if I got married and became the housewife I aspired to be! haha
Plus, I have changed a lot since then, in only 2 years. I still want to marry my boyfriend but my rose colored glassses are in the garbage and I think thats a good thing!