Is this through Office 365 where you go to the registered account and it offers to install things locally taking up one of the licences?
I’m not too sure if it pre-configures Outlook in that case, but you should still be able to manually add an account in Outlook. File -> Add Account, and just fill in the details of your account.
The licence doesn’t restrict it to one mail account, it’s just about what you can install where.
Having installed it from his account though you might find the Office apps are defaulting to logging in as his account. As I say, with outlook you just add your own account. In Word, Excel etc it will show the account logged in on top right of the screen and you should be able to change that. You will need your own MS/Hotmail/Outlook account though.