Major Cat medical problems....Help!?

I have recently had my cat into the vet several times because of a couple of issues....He is a young cat and has over the past year lost 4 pounds....2 of those pounds he lost in the past 5 months. He has had cronic diarrhea since kittenhood which no one was ever able to figure out. About 2 years ago I changed his diet to a hyperallergenic one (i.e venison, rabbit, and duck). He has done remarkably well on it as far as the diarrhea goes. Due to his weight loss we recently ran bloodwork, urinalysis, maldigestive profile, and have even done an ultrasound. Nothing really showed up except some small intestinal bacterial overgrowth. I started him on antibiotics but they gave him diarrhea. Next we gave him a B12 shot and a shot of Depo Medrol. Ok so now this is where it gets weird...since then he has been acting bizarre. He had diarrhea overnight, he has been very lethargic, and has been constantly licking his lips and has developed a strang facial twitch around his mouth. His pupils are also always dialated. He is just not himself. Oddly enough he also has a weird obsession with water (and that was before the Depo shot). And no it's not diabetes...tested! Kidney failure...NOPE! The hair on his back is also thinning. This cat has so many odd symptoms that make no sense. So as you can see I am concerened....looking for any input I can get. Oh and I should mention that he is a Ragdoll....PUREBRED!!!! So of course that means lots of issues. Thanks for your time and I hope you have some ideas.