Does anyone know where to go to start this quest? I talked to him, but I don't think I picked it up. Does anyone know what to do, or have a link to a walkthrough? I really want his magnum without killing him!

I remember reading something before release about a quest at one of the casinos, I think the Topps or whatever it's called, where you try to find entertainment for them. Since he at least claims he's good with his guitar, that may be it. Don't quote me on saying that this IS how you get it, but it's at least a good lead