I know the law on trespass is different from state to state and from county or township. But for the most part, if it's open land (not fenced or posted) your not trespassing. Unless your told by the owner, and that might need to be in writing. That you have been put on notice, that you don't have permission to be on this property. Now his not the owner, so technical you were not given a notice. How would you know who this guy really was anyway, legally. Even though it probably was the owner's son.