I would want to go knock on his door too in this situation. Call him out and see what his reaction is. Bring a couple buddies with me, and let him know people are watching him. My only concerns would be 1) Was he just some guy walking his dog? 2) If he is a sex offender, does it somehow egg him on that I'm standing on his porch calling him out? Does that prompt him to go after my wife and kid, if he can figure out which one they are?
I have been in a somewhat similar situation. I went to a park by my house to jog one morning, and I noticed a guy sitting in a beat up van in the driver's seat in the parking lot facing the park. Stereotypically speaking, both him and the van were poster children for sex offenders. He appeared to have a magazine open across the steering wheel, but he was watching the people in the park intently. He was parked right next to a sidewalk, so I slowed down and walked by him when I got in front of his van. He gave me a creepy smile and looked back at the park. It was 6:30 in the morning. Who comes to the park to sit in their van, read a magazine, and look at the park??? I called the police when I got home, but they pretty much laughed it off. Houston PD doesn't even really respond to gunshots, much less a "suspicious looking person". I also drove back over there, but he was gone. Thinking back on it now, if he would've been specifically watching my wife and kid and a police dispatcher gave me the info that he was a sex offender, I would've likely done something stupid.
