If you only want his email address seen by the recipients, the returned emails will go to his account no matter how you configure it. Let's list the possible options according to the email status:

Stage 1. The email reaches your email server, at this stage, your manager hasn't received it to his desktop, the administrator can filter this email to your account with some rules. This is the best option to me, because your manager doesn't have to know the emails at all.

Stage 2. If you can do nothing at stage 1, either you or your manager will receive the emails to the desktop. He can setup a rule in Outlook to forward a copy to you. You can also receive the emails directly with his account, by this way, you need to leave a copy on the server, otherwise you get all his emails.

So the solution will depend on the email server, what email server do you use? Do you have an admin managing the server?