You would have to have access to the same computer that he uses to access the account but one option would be to install a keyboard tracker of some type. Actual Spy is one that I use to keep an eye...
Really, it depends on which mail service you are using. Mostly, however, you click "New" or "Compose", then type in their email where it says "To:" or "Receptaints email". then in the big box or the...