Why on earth do cats fart?

I found a cat last christmas in my back yard... i think someone dropped him off b.c no one has tried to claim him yet..... he's a sweetie though and aged around 1-2 years. anyways... we had him for about three weeks before we realized something was weird.... we just thought someone had farted and wouldn't own up to it... n then one day it smelt perfectly fine, n my sister and i were sitting watching tv and smelt the nasty smell again, we knew our dad and mom wasn't home and both knew it wasnt us.... and we looked down and realized it had to be our cat..... his farts REAK like no other.... his poop doesn't even smell that bad, could be the litter covering it up but still...... why is he farting such a horrible stench..... is there anything to help his smelly butt???