Maybe before you bought the dog, he was fed some chemicals or something he isn't supposed to eat.
GET YOUR DOG CHECKED OUT RIGHT AWAY. My dog also developed a "chemical odor" to his breath. His appetite was poor and he had lost a few pounds; was lethargic and sleeping more than usual; and his coat was dull and he was itching a lot. Took him to a vet (quack) who said he needed his teeth cleaned (NOT the case; my dog was less than 4 years old) and prescribed some fish oil to put in his food to give his coat some shine and help with dryness. One month later, my dog's belly swelled up despite the fact that he had lost weight, so I took him to a different vet who immediately diagnosed him with advanced kidney failure. We put him to sleep two weeks later when his stomach continued to grow and he was having trouble breathing. No treatment worked; it was too late. GET YOUR DOG CHECKED OUT BY A QUALIFIED VET IMMEDIATELY if your dog presents any of these symptoms. Kidney failure, unfortunately, does not usually present itself until kidneys are 85% gone and it is too late.
Take him to the vet for some blood work, it could be a kidney problem.
My dog has chemical breath too...took her to vet what a waste of time see how she goes..this has been going on for two months.