Suit for rent on a building.

What state are you in? Some states have a statute of limitations (prescriptive period in Louisiana) that you have to pursue a claim within one year from the date of the incident.

If it has been a year, chances are he cannot pursue it.

Did your boyfriend's dad live there for a year without paying? Seems rather odd to sue for a whole year of rent. Sounds like maybe your boyfriend's dad moved out early and the landlord is trying to sue for the remainder of the lease.

1) If the lease was clearly expired, and was not renewed by both parties, the landlord has no case.

2) If your boyfriend's dad left before the lease was up, but then somebody else moved in later, the landlord can only sue for the number of months that the building was unoccupied.