Don't you just hate it when they change the rules on you in the middle of the game?
It would seem that you are in the driver's seat, if you already moved in and he accepted the first check.
But he can make life a little miserable for you, and,
You're going to have to pay it eventually anyway.
When it comes time to move out, you'll be glad that you had that money in the bank, (so to speak)
Just make sure that you make a copy of the checks and have him acknowledge it in writing, (you already know that you can't trust his word!)