Well, if that isn't quite the sound to be asking for. My uneducated guess would be that he's using an effect of some sort. I really can't think of any other way to produce such a sound as it doesn't seem like something that be produced so naturally. In the possibility/probability that I'm wrong it might be in your interest to find a video of him playing the solo and see if he does anything interesting. I don't know of anything off hand that would make such a sound though.