If it's transformative, it falls under fair use. I'm no lawyer, but I have experience with fair use. This is a parody, and falls under fair use laws. He took the clip, and changed the dialogue using his own voice (or someone elses') to transform the clip. It's the same thing with movie reviews, game reviews and book reviews. You can quote a passage from a book, show a clip from the game, or show a clip from a movie in order to review them. Same with parodies and covers. With a cover, you're using the lyrics and music and your own voice. With a parody of a song, you use the music, but change the lyrics usually (or if you're misheard lyrics, you just change the part that's always 'misheard' and use the video clip). This, I feel, falls under the parody portion of fair use. He probably should credit.