How about just agreeing to go to prom together? He is trying to stake a claim here for the future - he's afraid he'll loose you during college. That may well happen. Don't go off to college with your hands tied behind your back - stand firm about not accepting a proposal until you are out of college and ready. YOu just aren't in a position to make that kind of a decision now. You need to go away to college when the time comes with an open life - to accept change in yourself and others, and to meet a lot of new people. If your relationship continues (most don't, I'm sorry to say - but almost without exception it's for the better anyway), you will have lost nothing. If it ends, well, that's what was suposed to happen.
Don't lease your future so young. You aren't ready to marry, so planning for that is way too premature.