I will look into this approach, I wasn’t aware DMCA works on Amazon listings.
DMCA works on any Internet property operated by a US-based entity. Amazon.com is operated by Amazon US. You might run into issues trying to DMCA Amazon.com.uk but for the top-level main US site you're fine. Note that you must not overstate or misrepresent your case in any way, and you are strongly advised to seek a lawyer since this no comment on this post should be considered legal advice.
They might just remove his account too if they see it as a legal threat? I'm not a lawyer.