He won't stop bullying me !!! Help !!!!!?

Ok, before i start, please don't say that i'm a p*ssy or a wimp or anything like that because i'm not and i never let anyone push me around, but this case is different. There is this kid, evan, who keeps bullying me everyday by having his friends hold me down as he farts on my face, nose, and mouth. I work out n stuff but his 4 other friends overpower me when i'm by myself so he keeps doing it. It smells like s***. I've asked him to please stop it but he laughs and keeps doing it. I'm 18 so i can't really fight him, (which i really want to do at this point.) Its really grossing me out, and he knows i'm grossed out by it. In all honesty, why the hell would he do this ? Does he think its funny to have me smell that ? please help. I know this question is kinda idiotic, but i want this to stop i mean...seriously, i'm 18 and he's 15, this is getting me grossed out...help !!! I know ppl say to tell but its so idiotic to say to someone "he keeps farting on my face" please help, what should i do ?