The Meaning of 'Man'

I ran into a guy I go to college with the other day. We've been on break since before Christmas. I'm starting my second year back in school next week. And I'm 60-years old. So I run into this guy, I roll down my window and say, "Hey man. How are you doing?" He goes, "Hey, man, I'm doing alright."
"Did you have a good Christmas, man?"
"Yeah, man, it was cool."
"Hey, man, are you going to be back in school next semester?"
"Oh, yeah, man. I'll be there."
And this went on for about five minutes. When I left, my four-year old who was in the back seat asked, "Daddy, why did you call each other man?"
So I had to explain the meaning of man to my son. It really isn't easy. I couldn't tell him it's just a greeting like, "Hey, dude!" Then he will want to know what dude means. Next thing you know I'll be in over my head. And I hate it when that happens.