What was wrong with my cat?

I just recently (earlier today) lost a very special cat- he was a feral kitty with an age assumed of around 3 years. I work at a veterinary hospital and before he found his forever home, he was up for adoption for about 2 years where he has had a long list of health issues. He first started out having diarrhea and soft stool which we tried with courses of metronidozle and prednisolone, and there was a slight improvement but not enough so we took him off of it and was just a reoccurent thing along with fur loss due to allergies. We allergy tested him and found out he was allergic to almost everything (mainly outdoor, very little food allergies) and we did an ultrasound which showed a thickening of his intestines which led us to do an exploratory surgery and send out a biopsy and it came back as IBD which suprised us because we assumed that he might have cancer. After the expoloratory and just prednisolone and a diet change to royal canin PV dry he was doing amazing- all his fur grew back (he also had ringworm a couple times prior), his appetite was great, he was the most playful cat, etc. We finally were able to adopt him out to a good home and he was only there for 2 months and had to be hospitalized twice due to him being sick. The first time was 2 months ago, and his entire head was bald and scabbing so we sent out a culture on him which came back positive for ringworm, but because his immune system is so shot it was hard for him to kick it. But the ringworm was the least of our concerns because he also wasn't eating and had lost a pound while he was in his new home. We did bloodwork/urine/etc and all came back normal, and he started eating again so we sent him home. A couple of weeks later he came back with an upper respiratory infection and his head lesions where the ringworm was is now raised and bumpy and super irritated, he had lost more weight and was now sparking a fever which for about a week, was resistant to antibiotics. His bloodwork wasn't horrible (off the top of my head I don't remember exactly), but he wasn't eating and had lost more weight. He was super congested and had severe nasal discharge. Flash forward a week- congestion is gone and fever is down but still not eating and consistently loosing weight. He can't have wet food so hand feeding was difficult because anything besides PV dry causes diarrhea and severe GI issues/vomiting. We were grounding up his dry and soaking it in water but hand feeding was very stressful and often would cause him to vomit, so we switched to handfeeding chicken broth in small portions, slowly which he liked. We were also using a nebulizer on him, and washing his head with a shampoo of sorts to help the irritiation. He was also on a long list of medications to help his issues. His head was looking better, but when we tried to pull blood on him (which wasn't stressful- he was good for it), afterward he started open  mouth breathing but stopped and then was fine. Flash forward another week, he had another spell of open mouth breathing during handfeeding and couldn't stop spontaneously vomiting so we put in an IV catheter. Flash forward to last night- very weak and exhausted, temperature of 96 even though he was on a heating pad all day, IV catheter needed to be replaced but his veins were all shot and not functioning properly, bloodwork was super icteric, low glucose, etc. This morning, flat out in kennel, temp of 92, crying out in pain. Very heartbreaking that we couldn't fix him and find out a diagnosis that we could treat. I'm just looking for a little closure, any idea what could've been wrong with him? 

I am not a vet but it certainly sounds like a genetic problem with his immune system and probably autoimmune problems as well, in the end just not compatible with life on this planet.  I'm sorry for your loss but believe he's much better off at the Rainbow Bridge and out of that failing body.  What a hard life the poor guy had! 

Damn, that sounds brutal. I have no input on what may have been wrong but i just want to say that you sound like a fantastic person! All that effort you put in to try to help him out. Truly outstanding. Im sure he appreciated it. Sounds like he was in the best possible hands he could have been in regardless of the unfortunate outcome.
Im sorry for your loss. I hope you can find closure in knowing you did all you could for him and maybe from knowing that some random stranger on the internet thinks you're awesome!