"M--may I help you, sir?"
"M--May I help you, sir?"
Also, would a hyphen work instead of a dash (just out of curiosity)?
Example: "M-May I help you, sir?"
Aha, I see what what you mean now.-M-m-m, may I help you sir?
But I don't think you can have this in written English.
Double hyphen is also unheard of.
That's not supposed to be a double hyphen, but thanks anyway.