Question

Can the Word "Dear" be used by a man to address another man?

Answer 1

Yes you can! Two men can address each other as that.

Answer 2

It's a word that, in modern speech, is almost never used. The only time I ever use it is in the first line of a letter or e-mail (and even then, relatively rarely). And the only time I hear it is when someone's reading a letter aloud or absentmindedly says "oh, dear" because they've realized they made a mistake. Ironically enough, it has fallen out of favor as a term of endearment