OMG!! I am so sorry.
First off, are you sure, 100% that he is cheating?
If yes, then I figure stick out the lease with him if you can. Confront him about the truth of course, but tell him that you both need to be mature in this situation (Which sucks and will be hard).
Stay in the apt and then go to Case. No need to stay at Syracuse any longer and Case is a better school.
That is sooo sad, what a jerk he is!!!