LMAO embarring problem in one of my cles?

In one of my cles some boy keeps farting but whenever someone mentions it he acts like he doesnt know what everyone is talking about but we all know its him cos it comes from his direction.. i know it sounds disgusting but how do I tell him to stop cos he smells really bad and once the teacher told everyone to leave the clroom cos he thought it was a stinkbomb .. it was quite hysterical actually. !