What should I do about this?

This guy I've been talking to for a while asked me to go out on a date with him on Friday. He asked me on Monday. We've been texting back and forth non-stop for a few weeks and talking o the phone every night. Today, I was driving home from work, and saw him walking with a girl in this place a few minutes from my house where there a bunch of restaurants and shops. She definitely wasn't his sister (I've met his whole family), and definitely not his cousin. They weren't holding hands, but it seemed like they were more than just friends to me. It's kind of weird because I don't want to say anything to him about it and have it seem like I was stalking him or something. Do you think he's a player? Should I cancel our date?