OMG- my 11 year old son is so stinky now!! Seriously, on car trips, the back of the car just REEKS- but he is more 'silent but deadly' so all I can do is call him on it AFTER he's already done it.
On one hand, this is a natural function of our bodies. Everyone does it sometime or another. But- I always tell my son that if he KNOWS he is gassy, it is his body telling him he needs to go to the bathroom and that is where he needs to be! It is much better to go and - pardon the expression- 'fart it out' in the bathroom and see if you have to go, than to keep sitting in the living room watching TV and doing it.
He also seems to be under the impression that no one else in the room knows he is doing it!! I tell him that we can all smell it and tell when he has passed gas, even if it wasn't noisy. I tell him that you should always just say "Excuse me" or "pardon me" whether you think people heard it or not.
If he does it and doesn't excuse himself, I call him on it right away. We are dealing with some hygene issues now, and this seems to be part of it. I am talking a lot about how if you don't brush your teeth every morning, clean your fingernails, clean your ears,etc. - PEOPLE NOTICE IT.
The whole 'farting is hysterically funny' thing seems to go along with this. I will send him for a 'potty- time out' if he does it! Its sort of embarrassing and I tell him that we don't want to sit there smelling it and if he thinks its so funny, he can go laugh about it to himself in the bathroom!
We've also been talking a lot lately about 'just because your friends and you think something is funny, that doesn't make it 'MOM FUNNY'. I tell him he is plenty old enough to know how to behave different ways for different occasions and places. For example- he would be horribly embarrassed if he farted in front of his teacher, or at a fancy restaurant with his grandma. So he can learn to hold it or excuse himself in front of adults and keep the potty humor for his pals who appreciate it.
hope this helps- it seems to be an ongoing thing here!! Good luck!