My DS, who is 4.5 is the exact same way. Right down to the Britney spears and telling us the genres and if the song is explicit. Your description is eerily like my DS! What part of the country are you in, our kids could be BFF's! One time he was singing womanizer so loud in target I had to threaten to take away the iPod! It's ridiculous! He has track lists from all of his favorite CDs memorized. Right now he actually loves gorillaz, gee, that's appropriate! He does know he cant listen to explicit songs, but there are some that don't have swearing that talk about inappropriate things. That reminds me of his beck days.
He has one million toy instruments, he has always loved music. My inlaws are music fanatics too, everyone, including dh played more than one instrument, so I really think some of it is genetic. The rest I figure is a gifted quirk.