Always capitalize polite forms of address like sir and ma'am (or madam) in a salutation at the start of an email or letter. Also capitalize honorifics like sir and dame and titles like madam and miss when they appear right before a name or another title.

When Should sir be capitalized?

You need to capitalize sir when you are starting a letter or email. You also need to capitalize sir if you are using it as an honorific before the person's name. In every other case, sir should be lower case.

Should sir and yes sir be capitalized?

"Yes, Sir!" Replace Sir with Mom or Dad and it clearly should be capitalized. I've seen "son" written both ways (with or without caps). A grammar site I use says to capitalize when the word replaces a name, but not for nicknames (e.g., sonny boy).