(if I'm not mistaken, the original creator never said how he got this, but he says it's canon, if I'm not mistaken)
By Ticcing his Toby till he proxys
Personally, I think since he grew fond over the taste of flesh; I think he canniblaized the side of his mouth
From the horrific crash
Chewing through his cheek
I think it got burnt
Either chewing through it or the car crash, probably chewing through it, I do that too but not completely through it.
What do you think?