I can't promise how effective it is but Google Pixel phones have a built in scam detection feature in their dialer app. It uses a local LLM that runs on device and analyzes the phone conversation. Here's their support doc: https://support.google.com/phoneapp/answer/15654065

It likely only works for US English phones, and of course if they get him onto other platforms like whatsapp or signal then it's no help. Sorry to hear you're dealing with that it's a big fear of mine as my parents age.