Enter his email id in settings to link your ids and if it is already in use in this application, they will probably let you know the same. Or you may contact support.
I don't see where to put his email to see if he's already registered. Can you take a screen shot and show me please?
How do I add a screenshot? I see no option in the comments.
Goto my account >> scroll down >> relationship information >> your partner's email id and partner's invite code. If they are there, then his id is connected to yours. If not, ask him to connect with you.