Lots of people say "y'all" when referring to a single individual.

In general, these are not people who much care about grammatical precision in their speech. It is undoubtedly "incorrect" in some technical sense, granted, but then any context in which "y'all" is used at all is emphatically informal and cooloquial, to say the least, and this usage is fairly widespread.

In other words, this is not something the songwriter came up with on his own, and it is certainly not something that would seem incongruous in the folk culture being portrayed. It's quite natural and usual to address an individual as "y'all."