How far does he take it, and have you noticed this type of thing across the board?
I know this type of thing is attributed to certain personality types. INFJ in Myers Briggs for example, tend to blend in in groups, because they aren’t going to do anything over the top, and tend to be this way one-on-one with people. Not necessarily copying people though, it has more to do with body language, tone and gestures. Like if you’re outgoing, it may bring out that side of them. But if you’re quiet, they’ll stay low key too. That kind of thing.
However, actual copying,, especially in things that have to do with identity, is a characteristic of bpd/ borderline personality disorder. Single White Female is a movie where they showed someone who did this to the extreme, for example.
I’ll leave this here for you. Not so much about the diagnosis, because if there’s alcoholism, you don’t know how a person is without that, but just regarding the behaviors. Maybe these behaviors sound familiar?
(The website is -thriving is the goal- “borderline personality disorder and mirroring” is the article):
https://www.google.com/amp/s/thrivin...mirroring/amp/