He claims to have a gag reflex that when he opens Michael's stinky diapers it makes him gag and cough and choke. I think it is all a drama act to get some attention and help from me (just my speculation).
Now, when Chris changes a poopy diaper, he takes his shirt and pulls it up over his nose and mouth so there is a barrier between him and the stink. I still think it may give Michael a complex about pooping. :)
And for the record: I think it smells like roses!