but be prepared for him to flip out. When he does you need to rationally explain that although you still care about him you are not going to break your plans, and that you two are no longer together so you have every right to move on if it's what you want to do (can help him out by telling him you and your date are only friends, if it's true). You can also add that you are not ruling out that you two may get back together, but in the mean time you are going to make the most out of college and life.
I'm not sure where you two are at in your relationship or who broke up with who... so adjust to fit the situation of course
Whatever you do please do not break your date it will not only make you feel bad, but it will hurt the guy's feelings and leave him scrambling for another date as well.