Piecing together what he's said from what I can recollect because as you know, he blabs too much, he's said before he's 25% Italian (although he could be lying and it could be even less) and he's also said before that he's Italian on his mother's side, which is confirmed by the sauce recipe which he said he got from his mother and her mother, hence the Burnell surname too. So more than likely its his grandmother on his mother's side that is the full blooded Italian. This isn't even counting the possibility his grandmother could be American born Italian too so a genuine person from Italy could be way up his family tree. Either way if his grandmother was born in America or not, the whole Italian thing is a massive reach. People say he's got more Polish blood than Italian.
I remember some of the pizza DSP tries it and it was like a New York or Chicago style pizza like that and he talked about how its nothing like some good old genuine Italian pizza then he started putting on a fake accent and doing the hand gesture thing when he was saying something like "Genuine Italian Pizza righta from the oven" and shit.