Dear Bundy,
Remember that a bird in the hand is worth two in the bush.
Before taking legal action I would amicably discuss this with the tenant so that he fully understands his responsibilities and that you are really trying to help him. (Good to always play good guy bad guy and say that you will have to "talk with your people/partners/bankers etc etc." meaning that you don't have to make a decision on the spot if it's not in your best interests.)
If a tenant finds you a good tenant to take over the rest of the lease then you are also in a good position to raise the rent for the new tenant when the lease agreement expires in six months. Otherwise it may be in the old tenants best interest to subsidise the rent for the new tenant to ensure they take over the lease.
Overall I would use this methods in discussions with them because if you have to take legal action at the end of the day the person that makes the money is your solicitor and should be used only as a fallback position (Or only as a short scaring tactic).
Cheers,
Sunstone.