landlord

Hi Guys! Have a question, going through some issues with our landlord. He is somewhat of a dishonest guy. I'm wondering if and where you have to report rent collection? Our building used to be rent stabilized, but then he bought it out. From there there is no record of rents collected. I am wondering if he could be collecting all our rent but not paying the correct taxes? I can see the taxes on our building but can't figure out the math of what he has deducted...not sure if this makes any sense....but any insight if anyone knows anything about this it would be great!