New York City Tenant Question

I am looking to purchase a two family home in Queens. One of the appartments is occupied by a 72 year old man who has lived in the unit for 50 years. He is paying $550 per month rent. The market rent for his unit is $1500. I know that this property is not subject to any rent control or stabilzation. The realtor said it may be a problem raising his rent him as there may be some statute existing regarding long term residency. Any advice on this would be much appreciated.