i agree with everone else its the food - however i have a staffie and they are quite renowned for disgusting bums !!!
I found that it my dog is ever so much overfed - he stinks to high heaven regardless of his food - so whilst i think his food isnt the ideal, perhaps try decreasing the amount - or splitting it into two meals so that he has more time to digest it properly.
I have never used them myself but have heard that there are
these rocks that you put in their water bowl and its meant to really help with windy dog butts!!!
I think they are sulphur - I guess its worth a try!
Dan+Dogs. xxx