This does seem terrible. Can anyone suggest a way that we can help this guy? (assuming this can be validated - though I'm not suggesting it's not true)
Do nothing. He doesn't need an Internet cheering section. He needs a lawyer, and an extraordinarily straightforward civil suit.
Anything a net groundswell could possibly accomplish is only going to complicate the very simple "They falsely alleged copyright over my works. They ignored my reasonable attempts to correct their misunderstanding. They then contacted my clients and called me a thief. I have suffered demonstrable damage to my business and professional reputation as a result. This behavior is tortious." narrative his lawyer will be telling the court.
Anything a net groundswell could possibly accomplish is only going to complicate the very simple "They falsely alleged copyright over my works. They ignored my reasonable attempts to correct their misunderstanding. They then contacted my clients and called me a thief. I have suffered demonstrable damage to my business and professional reputation as a result. This behavior is tortious." narrative his lawyer will be telling the court.