Have you had his teeth checked out? Our boy Chuck who passed away in July had the worst smell during the last year of his life. At first I thought it was due to bad breath, however, no matter how many times I brush his teeth a day, the smell just would not go away. It smelt like a dead animal. We took him to the vet, and it turned out that he has two rotten teeth, and the condition was so bad that part of the bones that were in the sinus area were rotted away as well. The vet removed the teeth, and even after multiple surgeries, the gum tissue around those teeth would not heal, and thus leaving two holes which packed in food and hair all the time. We had to use saline solution once a week to flush out those holes; otherwise we would end up with dead animal smell in the house. Chuck's condition was due to the negligence of the first vet that we went to, who didn't find the problem, and therefore, delayed treatment for months. Also Chuck was a senior boy with deteriorating health and that was the main reason that his gum would not heal after surgeries. But once we have the bad teeth removed, and keep the holes flushed out, the bad smell went away.