> I was always taught it was 'Master' for unmarried and young men.

I remember when I was a kid, bank correspondence would address me as MASTER. I found it amusing.

I don’t think they do that anymore though??? I think our son has always been MR to them, even when he was a 5 year old

You're damn right it is

And when was the last time you heard someone called master in a newspaper or actual life?

"Master" is for a male child. (And has, unsurprisingly, fallen out of favor.)

And when did you last use that instead of Mr?

Even when the last name is "Bates"?