7 mo. old kitten stinks up the house...?

he is a Maine coon. we got him back in January. any change in his diet and he will have the runs for at least a day... so he is not given any table scraps. but we currently have him on a mixture of dry kitten food and dry adult hairball ( no kitten hairball) and he will get some canned food. and the same treats. but even if the litter box is freshly cleaned and he goes poop in there , buries it and gets out of the box you can smell it across the house. we have tried different litters to help with the odor and they don't do a thing. so could it be that he just has a super sensitive stomach and the food that we are giving him isn't easy enough on his stomach? I mean it is pretty bad when he comes running out of the room after he poops! HELP... the stench is horrible!