What your describing is probably a bad tooth, gingivitis, or some form of dental disease.....Statically, most
dogs by the age of 3, have some form of Dental Disease.....
You need to get your guy to a Vet for a Dental.....During the dental, your doctor will be able to determine if there is an abscess or a tooth that needs to be extracted.....
Just with humans, dental disease is quite painful....That's why your guy is not feeling well....When it gets worse, he will quit eating......