He's not a City fan. They are simply his home team. We all have a home team and some of us follow that team more closely, some less. Nick proved many times that Juve is the only team he really cares about. He supported Juve when we played City in EL last season.
Stop giving him hard time and be his mentor. Plus, despite your disagreements in the past, he appreciated your guru work with Vidal and he congratulated you. A little bird told me that he was actually impressed by you and he regrets not trusting you in the past