I dont know the specifics regarding the laws for your area but generally:
If the tenant does not give proper notice, they are responsible for the rent for the period of which notice was required. If they should have given 60 days notice, they are responsible for 60 days rent (and any related utility costs).
In your case the tenant signed a 12 month lease and then broke it. Generally they are responsible for ANY lost rent until you get place re-rented. (upto the end of the 12 month, lease, period)
You may have to show proof of attempts to rent it during this period (at a hearing). (newspapers ads, internet ads ect)
The tenant would also be responsible for ALL costs asociated with re-renting the apt.
advertising, credit checks, milage, faxes, ect.
No you can not change the locks till the tenant has actually vacated the apt (even if they are staying in the apt and are not paying rent)
He can move back in and not pay you rent. He has not given you written notice of his intent to vacate and it will be your word against his.
I would say that since he has not given proper advance notice or a written notice that he intends to leave, should he leave, he is responsible for the costs listed above.
Did you collect a security deposit?
At this point, I would want him to go. He does not want to live there and will be problems to you if he stays.
I would tell (dont put in writing as it may be used against you latter),him that if he leaves now (sept 21), even though he has not given you proper notice and legally owes you money, I would use his last months rent to cover some of these costs and release him from his legal responsibilities.
That means you use the last months rent to pay for October.
I would also let him know in writing that if he chooses to stay on, his rent is due on the first of the month and failure to pay it will result in eviction, a negative credit history and additional related eviction and legal costs.
Please keep in mind that I dont have ALL the facts and you should seek other advice. --99.234.xx.xxx