I returned to school a few years ago to earn the grades I needed to get into medical school; consequently, I'm the same age as some new professors. One of my acquaintances from a sports club I belong to is approximately my age and is a professor who researches something of interest to both of us. A conversation about his research lead to me volunteering as an RA for his lab.
Given that we were friends first, I worry that addressing him as as 'Dr.x' might feel weird for both of us. Moreover, it seems that few professors are comfortable asking to be called 'Dr.x' when someone asks them what they want to be called (even if they would prefer it), so I don't think asking him will be of much help.
Lastly, I don't worry about calling young professors 'doctor'; I wouldn't have much of an issue calling someone older, or even younger, than me 'doctor' even if we met outside of academics, It's the fact that we're the same age and that we were introduced outside of academics that makes it, for some reason, weird.
That said, using his first name doesn't feel right either.
I'm considering using his first name in private conversations and 'Dr.x' in public ones.
How should I address this person?