is it possible that my boyfriend could be hiding his encrypted emails inside of my email inbox with my email address and I can't read them? And if it if that is possible is there way that I can decrypt and read them?
Hiding emails in your inbox would not be a smart move. Email accounts are free, so there's no need to use another account to store them.
Any email that is stored on your account should be visible to you. Even if those emails are stashed on another folder (not on inbox, for example). You can access all folders and see if there's something suspicious. If you find something suspicious and encrypted, you probably won't be able to decrypt without the key.
If you suspect anyone is using your email account, change the password immediately. This is an easy way to lock anyone out.
But I don't think your boyfriend would be using your account to hide emails from you. Does not make sense.