Emails I read as my bosses delegate in my inbox showing up as "read" mail in both of our inboxes!

In Apple Mail, I have access to both my inbox and my boss's. (He only has access to his own.) We are often copied on the same emails. I only read emails in my own inbox, but if I read an email IN MY OWN INBOX that he is copied on, it shows up as "read" mail in HIS INBOX too. So, my boss can't tell which emails he's read and which he hasn't.

Can anyone help me solve this? It's a huge issue, and my boss isn't seeing very important emails. Going through and manually marking his emails as "unread" every time I read an email is not an option.

Thanks in advance.