Cleanliness is next to someone else

Kid is a fucking inconsiderate dick. Not to be petty, but start chewing and popping gum really loudly or something and tell him you’re “opposed” to chewing with your mouth closed. Or you could go the non-passive aggressive route and tell him directly that it disturbs your environment (and everyone else around him). It’s probably on purpose, though - he wants to keep people away. Your best bet is to explain the situation to the teacher and ask to change seats. Hopefully your teacher isn’t a dick too though. Some of those jerks just go mad with power and say no to all student requests just because they can...

TL;DR: FYL, OP

Well I hope he isn’t “opposed” to having no teeth, because they’re gonna rot and fall out soon enough. Gross.