"He" would be the non-paid preparer. If he is paid to do any returns, I think he may need to sign as a paid preparer and possibly would need to renew his PTIN. You will also need to add him as a preparer. https://proconnect.intuit.com/support/en-us/help-article/partnership/adding-editing-preparer-informa...

Review https://proconnect.intuit.com/support/en-us/help-article/data-security/managing-access-rights-proser... for what "rights" to give this user. 

You will be the ERO, either paid or unpaid 🤔

Just as you do with office & home, you could find a way to transfer files between each other. "He" could 

If there is a "sale" of clients you both will need Form 8594.