Help- Any ideas

My moggy Kilo is 2 years old. He was a very mischievious energetic cat full of love and charachter. We had him since he was a kitten. When we first got him it became apparent that he could not chew. On taking him to the vets the vet thought that he was in fact too young to have been taken from his mother so for the first few weeks we had to hand wean him. We were advised to feed him cooked chicken as its easy for a kitten to eat.
Kilo blossomed into a great cat but despite every effort refused to eat anything but chicken but was happy had a gorgeously sleek and shiny coat and seemed ok.
At the end of Oct this year Kilo came in crying. On inspecting him t appears he had either been attacked or in a fight and I dont think he won. His whole body was covered in small marks and long scratches with one in particular under his front armpit causing him pain. I took him to the vets who checked him out and gave him an anti inflammatory injection. Although still in pain and a bit quiet Kilo seemed to get better.
About a week later I woke in the night to find Kilo having uncontrollable diahorrea. This was bright yellow in colour and extremely smelly. He could not make it to the litter tray and was getting distressed. I took him to the vets. He had a slightly raised temperature and the vet said that it could be stress.
This diarrohea went on for four weeks. Kilo had blood tests done which the vet said were normal. He had faecal checks for all the usual things and they were all normal. Throughout this period Kilo seemed to want to eat and drink, if anything was drinking more and in between seemed happy and was purring. He did however still squeak when he was picked up. We were not sure if this was due to some stomach pain or pain still from the fight.
Eventually Kilo had an x ray which showed a stomach that was backed up with diarreah at the rear end, in the middle was a large pocket of wind and the top end just showed food. Kilo had had vitamin injections and antibiotics, all which seemed to have made him better for a while. On the day he had blood taken he had not beenn fed so when he came back from the vets, having discussed it, I took advantage of his hunger and fed him cat food advised by the vet. He ate it and has been eating it ever since. Eventually the diarrohea started to fade and kilo started to return to normal. Then his back legs started giving way. He could not even climb up on the sofa and when he embarked on his usual run down the stairs to greet us when he came home his back legs could not keep up with his front ones and he would fall. He seemed to be ravenously hungry, but I didnt know if that was due to him being so ill for so long.I took him to a vet who looked at him and said that he thought it was neurological or possibly down to a deficiency caused by his diarrohea. He gave him a steroid injection and asked me to call him in a few days to report on Kilos progress. It seemed to have some effect and several times the first evening Kilo jumped up onto the bed which he hadnt been able to do. He seemed to be b etter but it wore off over the next few days. I did notice a small scab near to his anus, just tucked under his tail and if I touched it he cried. I mentioned this to the vet on my next visit. The vet checked his reactions etc and decided to do a blood test for FIP, FELINE HIV, one other incurable disease and a parasite called Toxoplasmosis (i think). The vet said obviously three of these diseases are fatal, and the parasite prognosis is dependent on the damage caused. However if all these results come back normal he is suggesting an MRI.
I dont know what to think or suggest. One other vet I saw was going to do anotrher test for diabetes ( not testing for glucose and started with f) but when I saw my usual vet he said he had already tested for it.
Im really worried about my Kilo boy and although he still likes having his hin stroked and a cuddle, the life and spark has gone out of him and he mainly sleeps all day. He does want to go out but I cant really let him out as he cant get about. I did a couple of weeks ago in the freezing temps here in the Uk and after searching for him for three hours found him stuck under a neighbours decking. This is the cat who would jump from the decking, onto the banister onto the shed roof, leap onto a satelite dish, bounce of it and through the railings of the first floor balcony to get in if he wanted to. I need some help guys. This is my one true friend I have at the moment

Kilo blossomed into a great cat but despite every effort refused to eat anything but chicken but was happy had a gorgeously sleek and shiny coat and seemed ok.

Just to make sure I'm understanding correctly, did Kilo eat nothing but cooked chicken for the first 2 years of his life? Do you happen to know if he was also eating prey that he caught while outside? Cause if not, taurine and calcium deficiencies come to mind. Cooked chicken without supplementation would be a terribly unbalanced diet.

I took him to the vets who checked him out and gave him an anti inflammatory injection.

Was he ever given an x-ray or an ultrasound to check for internal injuries?

I took advantage of his hunger and fed him cat food advised by the vet. He ate it and has been eating it ever since.

What exactly is he eating now (brand, wet, dry)?

One other vet I saw was going to do anotrher test for diabetes ( not testing for glucose and started with f) but when I saw my usual vet he said he had already tested for it.

That would be a fructosamine test (average of blood glucose over a few weeks). Do you know what the result was? When was that done in relation to everything that's happened (after the leg weakness? before or after the steroid injection?)?

Hi
Thanks for your replies.

Yes for the first two years of his life Kilo would only eat cooked chicken. He has had regular check ups at the vets and despite his diet always seemed fit and healthy.
He is now eating a brand called Applaws. It is very expensive but is a specilaist food that contains natural meats but al the other elements a cat needs in his diet. He is eating like a horse.
He did have a xray when he had diarrohea mainly to check to see if there were any foreign bodies in his body but it did not show any and no obvious brolken bones.I have found a tiny scab on the underside of his tail just above his anus which is sore to touch but the vet thinks its nothing.
At the moment they are not doing the fructose test but testing for the main diseases i mentioned in an earlier post.
I have been trawling the internet and agree that there could be some kind of defieciency. In the last few weeks he has had a vit B injection. I have considered giving him some kitten milk which is high in calcium and vit d and e

Oh and as far as I know Kilo has not eaten anything he catches. He seems to see it all as a game. We live on the edge of a lake and he does not even try to chase or catch the baby ducks or geese etc. I have only ever actually seen him with a baby bird that looked so small it had probably fallen from its nest and all Kiklo wanted to do was play with it. I know he didnt eat it.

The first set of blood tests kilo had doen were done at the same time as the x ray and this was where his glucose levels were checked. It was actually a different vet that mentioned the fructose test but the usual vet has now ruled it out