Why "Boy"?

It was a fairly common nickname in England--still is, probably. I doubt if there was any particular significance to it. An aunt of mine was the only female in her group of siblings, and we all called her Girlie, even when she was grown up and married with kids of her own.

I suspect it is because Barrett was under the age of 21, which in the England of 1961 was the age of majority when one became "an adult". It was also a term of endearment among gay men for a younger lover.