***** NOT A TEACHER *****
I am not disagreeing with the excellent answers that the OP has received.
I understand that learners come here for modern and natural-sounding English. And that is exactly what they receive from the teachers.
I only wish to contribute my OPINION as a non-teacher member.
Regular, natural-sounding English: "Excuse me, sir, but may I ask you a question?"
Formal, very respectful English: "Excuse me, sir, but may I ask a question of you?"
The second sentence, in my OPINION, shows respect and humility. The first one is too direct for my tastes if I am speaking to a superior.