Heh. I (didn’t) get a over 300 spam emails today.
As I say in that thread, I have a spamblocker (as part of my email account) that automatically deletes emails from blocked senders and domains. They’re just gone, and I wouldn’t know about them except for a report that lists the addresses, subject, date, and reason why they were blocked. If your brothers email provider has such a feature, he can set it to the ‘high’ setting so that he doesn’t receive them.
But it sounds like receiving spam isn’t the problem. It sounds like someone is spoofing his address. I don’t know if there’s any way to fix that. On rare occasions I get spam from ‘me’. I suspect that your brother’s (and my, and zillions of other people’s) address is on an email list that was sold, and spammers take valid addresses from the list when they spoof the Sender address.