Consider getting him Malwarebytes for Mac to detect some of the phishing sites. Not perfect but nothing is. If he is open to it see if he can use Fastmail and

Consider getting him Malwarebytes for Mac to detect some of the phishing sites. Not perfect but nothing is. If he is open to it see if he can use Fastmail and manually adjust the spam threshold in the settings. His old email I am assuming gmail can be forwarded to the new one. Both options cost money but all the free solutions I know of would confuse or frustrate an elderly person that recently got into using computers and Gmail's spam detection has gotten significantly worse in the last few years.