Being outside of the warranty period now is no big deal. If you took the car back to him for the problem within the warranty period, and it sounds like you did, then you should be all set. Assuming it is the same or similar problem. By letting him know of the problem during the warranty period, and giving him multiple chances to fix it during the warranty period, you have preserved your rights. He knows it. He is counting on you not knowing it.
As for documentation, he has it. He is probably, by law, required to keep records of the service he performed during the warranty period. He is also required to give you copies of those records. Ask him for it.
