I'm no expert, and am not recommending any particular copyright-related recourse, but the image submission agreement has a clause to remove materials from the service, and the license grant to Amazon is not specified as being irrevocable: https://www.amazon.com/gp/help/customer/display.html?nodeId=...

Total speculation here, but probably because common law trademark would have to be proven in a court of law. That is, he would have to sue the Chinese company, present evidence like newspaper clippings or testimony from customers, win, get a judgement, and present that to Amazon. Until then, Amazon looks in the USPTO database.

The very first step in Amazon Brand Registry is to tell them your US Trademark registration number. Without it, I'm unable to get through the Brand Registry website. And I can't find any email address for them - nor, any place to mention this to. That's why I posted that to my website's home page.