What does it mean when a young black man calls a young white guy the N word?

Usually it would mean that he's being friendly.

There is a movement, particularly among black athletes, to rehabilitate the word,

The prejudice against the n-word is fairly recent, dating from the late 1930's when Agatha Christie changed the name of one of her novels to "And Then There Were None."

http://en.wikipedia.org/wiki/Ten_Little_Indians

Prior to that, Great writers, such as Joseph Conrad and Mark Twain used the word extensively, 214 times in Huckleberry Finn alone.

http://www.gutenberg.org/files/76/76-h/76-h.htm

Christie, Twain and Conrad were not racists.

The Google ngram viewer shows the relative use of the term from 1820 to the present. The drop-off in the 1940s is striking. The comeback in print continues.

https://books.google.com/ngrams/graph?content=****...

For the foreseeable future, I don't recommend for white southerners like me to use the word, or expect the name of a prominent organization will be changed and abbreviated NAAN.

You would have to ask them to know for sure.