I'm guessing this is mutual.

I don't think Sean encouraged him to stay and this might be a face saving gesture for both of them since they're friends. For whatever reason he wasn't a good fit here. Say what you want about an OC here doing nothing - but it's can't be true. There's no way a head coach can spend every waking hour on designing the offense and gameplan. For further evidence - he has 2 former OC's who are successful head coaches.