I can see having his pants fall down while picking up the first case. But to not pull up his pants and continue to get several cases? Not cool. You do not expose your bare butt in public and do nothing about it.
OP, I don't think you did anything wrong. I don't even have a problem with your comment after rude lady cursed at you. I think your mother could have done a better job at schooling her expression, though. I wouldn't have said anything to the man, with the woman standing there. Surely, she could see the wardrobe malfunction, if he couldn't feel it. Saying something to him would have set her off, too. Although he might have done something about it.
I don't know about specifically asking for a manager but I think I would have informed the first employee I found, both about the bare butt and about being cursed out.