I waited for a response, but I'll go ahead with suggestions......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......
Don't let this go...A Dental problem left untreated can escalate to systematic infections, that eventually will damage the heart......
Please, schedule an appointment ASAP......Let us know what you find out.....Karla