I don't know, it sounds like he had a mild stutter to begin with and with proper editing it can almost completely be hidden. I think its possible for people with mild stutters to 'overcome' stuttering to the point where it can practically be hidden most of the time. However the same is harder to do if you have a severe stutter. Thats why I always when I see 'famous people who stutter', I think to myself, I can't relate to this...they probably had a stutter as a kid or have a very mild stutter but still identify as stutterers.