M.L.
My dog does this too, usually right when she comes in from outside, like this musty, sweaty smell and it goes away after she calms down. Maybe he is having gas, belching a really nasty smell, if the smell is fleeting than that's probably it? You can always try and change his dog food, go with a salmon/white fish one, just make food changes a gradual one so as not to upset his stomach. I worked at a vet and animal hospital and the white fish is good for sensitive stomachs and allergies. I know though, liver problems and such can also cause really bad smells, so if you are really concerned, take him to a vet for some lab work. There could be something going on with mites or in his eyes/skin/ears/teeth that you can't see but could be picked up with an exam.