I don't know if I would tell him right off the bat that you were recording it.. only because then you couldn't get any evidence.. in my head he'd be like "oh ok not that person, we'll wait for the next one." I wouldn't NOT tell him, I would just wait a few minutes, getting on the recording, you asking him to not come near you and when he continues, say "I'm recording this for record, please stay away." But that's just my thinking, I want evidence and for this guy to understand that he is in the wrong.
Though, it might also just be an ignorant owner who is trying to get his smaller dog used to bigger dogs by forcing him to walk by them and a simple "Hey dude, what are you doing? Do you understand, A, B, and C....??"
Commander Charly Titan (GSD)-15 Jan 2010