It certainly sounds like it would be to me., They lived with his father, then they left with their stuff behind. In most states no written lease or contract is required to establish this relationship. But, PA may define these relationships differently. The states I am familiar with would tend to define this as a landlord-tenant relationship ... unless, of course,the OP and her boyfriend were living on a couch and the stay was short.
In either case, this is almost certainly going to be a matter settled in a Small Claims or other civil court.
It is also possible that PA grants a person on whose property crap is abandoned - and NOT a landlord-tenant relationship - a relatively quick chance to dispose of it. I found the ten day notice rule to be rather swift, so they may have something similar for abandoned stuff.

