...That's pretty much the conclusion his parents have come to. Like I said, all involved parties recognize that the No One wants to deal with a tantrum. Even those (parents, doting godmother) who are obligated too. 
Have you tried carrying him in an uncomfortable manner?
When walk-away isn't an option, I have picked up DD by the waist and let her feet and head dangle. She gets so indignant at the position she is in, that she changes gears and wants down or to be held differently. And, she is willing to promise good behavior.
When my youngest misbehaved, I make him walk with me while I held his *arm*. Not his hand--his arm, as if it was a handle, and he was a package.
He hated it. It is a little bit demeaning, to be honest. Not so bad that I really felt awful, but bad enough that I knew it was unusual and negative for him. So whatever it was he did, he didn't do it again!
Re: leashes. When I was in junior high or high school, they weren't being sold as a product. But we went to the Iowa State Fair, and there was a woman who had a genuine dog leash clipped to her son in some way.
My mom saw it, and said, "look at that!" and made a beeline for her. I was SO embarrassed, and the woman looked really defensive. Until my mother said, "you are SO smart! What a genius you are! I bet people have been saying stuff to you about that leash, right? Well, you just ignore them, because you are a good mother, to give you child the freedom to walk around on his own, and to keep him from being scared if he gets lost. I probably shouldn't have interrupted you, but I just wanted you to know that I admire you."
The woman looked SO relieved and said, "thank you! that's exactly how I feel about it, but you're right, I've gotten a lot of negative comments."