Need some Advice

I am doing to just give the details, the first Discussion I typed timed out because it was too long, lol.

I have a 12 year old American Short Hair, 19 Lbs, indoor only.  Has had a senior check up with 2 follow up vets visits in the last 2 months. He is a special diet and has had tests to rure out that he  has diabetes because of his weight .
He has been licking himself bald, often resulting in bleeding. Vet said he has OCD and prescribed Kitty Prozac which leaves him being doped up. He is still licking non stop. He often jumps up and meows like something bit him (which it has not). He also had a recent steroid shot to help, which it has not. 
Two weeks ago he started to pee outside the cat box. I have had him for 5 years (daughters cat- new grand baby is allergic) and he has never done this. So now I have to baby gate him in the tiled laundry room where his cat box (and food and water of course) instead of him having the usual run of his kingdom. There has been no changes in our home to upset him.
I am extremely sad at the thought of having to put him down, but believe in quality of life. Can I have some advice or opinions. I apologize about rambling just trying to get all details in.

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