Severely misirable cat

My cat Chase is 2 years old.  He is neutered and has been since he was a year old.  He was declawed at the same time, front and back.  For the past 5 months he has been ill.  He started by losing weight.  He also started getting very strong smelling urine, more so than normal, and foul smelling feces.  He also seemed to start losing his appetite.  Then after a few weeks his hair lost it shine and started falling out causing him to vomit excessive hairballs.  He lost about 4 lbs in that time (about 2 months).  He was originally about 9 lbs.  In his bald spots--back of his neck behind right ear, along jaw next to mouth and along spine and chest area--he had small red bumpy spots in some areas and seemingly dry flaky skin.  I treated him for worms and ear mites as well as flea bath that treated fleas and eggs/ticks/mites the first month and that didn't help.  So the second month when he just seemed to be getting worse and after I had saved up I took him to his regular vet.  They treated him with FrontLine Plus and said he was allergic to fleas but that gave him som hydrocortisone creame for ringworm, just in case.  However, that hasn't helped.  He seemed to maintaing for about 2 weekd then then bald areas that had been regrowing hair started to lose it again and he was scratching and licking it again.  So I retreated him with flea bath and One Spot.I also got wound care spray for the sores on his neck and spine.  He has since gotten excessively lethargic and morose, he wont play or lounge in his usual areas.  He started to stay in the bathtub.  Then he ran away.  He has never been outside and is terrified of the outdoors.  Now he won't come in except to eat, usually every two days.  And when he does he only eats a little then yowls until he can get outside again.  I thought maybe it was worms after all since he has developed sores under his tail this week but when I opened his mouth to put the liquid wormer in his mouth I noticed that his front teeth, in between his canines, are missing.  There are slivers of tooth left but thats all.  He had teeth a week ago.  Any ideas what is happening to my poor baby?  Can I do anything for him?