M.G.
One of my relatives works on a pediatrics team - and she sees this quite a bit. There's a psychologist on the team too. They handle this kind of thing.
From just hearing her talk over the years (always in general terms, she doesn't share personal stuff), she has said the worst thing parents can do is turn it into a negative experience - so punishing a kid after (removing privileges) is reinforcing that it's something to dread. I get it - totally - that it's a consequence for mouthing off and being rude. But he will come to associate having a punishment with doctor visits. So instead reward him for not mouthing off - so say "if you can be pleasant or NOT mouth off at all to the receptionist, we'll consider doing .... afterwards". I know that sounds nuts - but you have to start turning this into a positive.
It's kind of the same thing with anxious kids (I had one). You don't say "It's no big deal it takes 10 min' casual way - to them it IS a big deal. You're diminishing their feelings - which to us, doesn't make sense, I get it. To them, it's a HUGE deal. They think you don't get it which further upsets them. Anxiety doesn't make sense - to them it's like a fright and flight (whatever that term is) response. Just remember that. It's physiological as well as psychological. They are not able to control it. They need some coping tips.
That's where a therapist can help. Even 2-3 sessions can turn this around for you. They do work with very specific anxieties. My kid had a very specific one. Coping strategies (breathing for us) helped big time.
But parents - our job was to be positive and encouraging and not diminish what they were feeling. Hope that helps.