My question involves landlord-tenant law in the State of: Illinois

I am currently abroad, and found a subletter for my apt. He agreed to sublet from Aug - Dec (paying full month's rent including Dec). He agreed both in writing (email) and verbally.

He is not debating that he agreed to pay all of Dec rent, but just told me he wants to move out Dec 1st and not pay Dec rent - he wants me to pay it to make things easier for him. His reasons are that it is too hard to move at the end of Dec, and he can't pay for two places if he moves early.

Is he legally obligated to pay? What are my options here? Thank you.