Adverse possession requires 7 distinct criteria in PA. In particular, your neighbor might fail on the exclusivity requirement (he must have exclusive use). Here's a useful article to get you started on this topic:
http://www.lgc.state.pa.us/deskbook06/Issues_Private_Property_Issues_03_Title_by_Adverse_Possession.pdf
http://www.lgc.state.pa.us/deskbook06/Issues_Private_Property_Issues_03_Title_by_Adverse_Possession.pdf