Goodness. Sorry you’re dealing with this, but this thread could end up being very useful for others. I wish I could help.
How do they react when you stroke their necks? And I know putting him in a pool would be a scary thing, but can he try to swim with you watching? I’m thinking...
I’m not even sure what else to add that hasn’t been suggested. I do also wonder if maybe others are picking on him or even trying to mate him, when you aren’t around to see? Mine all get along well too, but I occasionally catch some shenanigans going on unexpectedly.
Maybe review some camera...