Quoting Barnaby:" Lol yeah when he tried to explain it because I had NEVER heard about it before apart from in csi episode ... [snip!] ... absolutely disgusted at me but yeah if I don't ask then I don't know.. and I had NEVER head of a furry used in everyday life."
It's getting more common. Like you said, maybe he just wants to identify with something and your typical cliques just don't interest him.
He's a 17 year old boy. He'd be online constantly anyway, haha. I know it's odd that he's doing this and adopting this personality, but I'd be afraid of making him ashamed of it.
Have you seen gag or parody twitters? It's kind of the same thing. He's just adopting a personality on the internet to get views from like-minded people.
I'm sure he's a great kid, and if you couldn't tell how into it he was until you found the twitter, I wouldn't say it was that big of a deal!