“he was nobody” VS “he was a nobody”

Question

Nobody, as a countable noun, can mean "a very unimportant person" as in:

A man in my position has nothing to fear from a nobody like you.

But I don't quite sure about this example from the free dictionary:

I knew him when he was nobody.

Is it also correct to say I knew him when he was a nobody? Any reason behind this?

How about He is nobody VS He is a nobody?

Answer 1

To be nobody is to lack reputation and its prerogatives.

To be a nobody is to be one of the multitude of nobodies who lack reputation and its prerogatives.