Log directly into your email provider's website. (i.e. DON'T use any form of dedicated email app). Take a look there to see if there's a 'Spam', 'Junk' or similar folder. I strongly suspect that your brother's emails are hiding in there. (If they are, open one of them and click on 'Not spam', or whatever your provider labels that option as. Also ensure that your brother's email address is shown in your Contacts list).
If your brother's emails aren't in a folder on your email provider's website it suggest that they're getting blocked by a spam filter further along the chain. That can happen if an internet user is allocated an IP address which has previously been used by a spammer, so get him to change his IP address. (That usually involves nothing more than disconnecting the router from the phone line, waiting for a minute or two and then reconnecting it. If he goes here both before and after doing so he'll be able to check whether his IP address has indeed changed:
https://whatismyipaddress.com/ )