How do I leave my job if the contract says to give minimum 3 months of notice?

I have a part time job that I got when I first came to Japan this year. Because I was naive and in desperate need of working, I did not complain when my boss wrote in the contract that the minimum amount of notice to leave the job would have been 3 months. I asked about it, and he said it was just a formality.

I’m getting tired of it because he’s been a pain in the ass the whole time.

He used to ask me for a record of amount of worked hours, now he says he doesn’t need it anymore and he will pay me according to his records (useless to say, I got paid less according to my one ones).
I also used to live closer to my job and, although I used to take the train to get there, he never paid for my transportation fees because he asked for a monthly record but my job was on the same way as my commuter pass (so the record was impossible to get). Now I live further away, managed to get my record, and he said “giving a record higher than 3000¥ is not allowed” and refusing to pay. I also asked about this when it was in the contract back when I was hired and he said that was another formality.

I cannot deal with all this bullshit anymore. I am a broke student in need of a serious job, I cannot afford lawyers if he sues me or something. My school doesn’t help much either, sadly. But I cannot bear staying other 3 months under him.

What can I do?