I'll say this: if you were to try to forge a signature in such a way as to make it look as if it were signed in that "state of inebriation," it would most likely look very contrived. This one doesn't; it looks as if it were done in a somewhat casual fashion (i.e., not calculated). That said, there are certain key aspects of the signature that fit with Buk's signature, so that lends some support for the story being told. The upper case printing looks off though, but that might be chalked up to the aforementioned state. So, I'll bet it is genuine, but I'm not sure I'd buy it because it could be difficult to re-sell if there's doubt out there. It wouldn't have hurt to have edited the images to rotate them 90 degrees either.
I actually talked with this guy and believe 100 percent it's real. He got the "honored" privledge of driving Buk to a reading and he and Buk got completely fucked up on the night he signed it. It wasn't their last meeting and he has other Soundes-like stories to tell if you'd like me to invite him here. He's a teacher in a very small town these days and seems like a good guy.