Don't "let him out" of the lease. Write him a notice that says that you agree to terminate his lease in the event that you get a new tenant paying equal or greater rent than he was paying and that he has to continue to pay rent until such time as his lease is terminated and you will send him formal termination paperwork at that time. You are fully protected by the law unless you agree to cancel his lease now.
This is an example of why it's important to develop a policy BEFORE the situation actually arises.
I give my tenants two choices: pay an early termination fee or continue covering rent/utilities/landscaping until a replacement tenant is found. If they choose the former, I charge them a hefty penalty but then they can walk away with a good landlord reference. If they choose the latter, they run the risk of paying more if it sits vacant and their record will show they violated the terms of their lease.