Where do you work?
It's certainly very odd when you think that the majority of work places provide free tea and coffee and some will have soft drinks which also contain caffeine.
I could see how it might look a little unprofessional to walk into some kind of high powered meeting with a Red Bull, when everyone else is on a coffee or such. I guess it looks a little crass and like you are so worn out that you need to be drinking it.
he's not drinking it in customer meetings.
He's drinking it off site on his work break.