I was born in germany and I have never heard anyone call elderly men "Onkel". The only person I would call "Onkel" is my actual uncle and in that case I would still follow with his name ("Onkel Markus"). But this would be equally unusual as calling my sister "Schwester Lisa". So I usually don't call anyone "Onkel", it seems like a joke and is not appropriate when addressing a waiter. —
stimulate 1 min ago