Is using wifi stealing?

The guy who lives in the condo next to my mom's is a friend of hers. He works mostly from home nowdays. I figured out his passcode to his wifi and I been using it since then. My mom don't have wifi. Am I doing wrong? Am I stealing from him somehow? I mean its not like its costing him more by me piggybacking on his service, but I'm not sure what I should think about it or do about it. Am I a thief ? ?