I am Deeply sorry for your precious kitty....My prays are with you .God Bless!
Unfortunately his organs may have begun shutting down, I just lost 3 of my 17 1/2 year old kitties in the last 5 months. The last one Max acted exactely like your kitty but it came on suddenly, we knew his kidneys were failing but he was hanging in there, but then he walked into the kitchen, got super shaky all over and then lost use of his limbs, I wrapped him in a towel and took him to the er vet, I knew his time was up, these are my babies as I've never had children and it kills me to lose one but you have to do the final act of love and let them go peacefully with the least amount of pain as possible, do not let him linger because it's going to hurt you so much to let him go, I feel your pain but enough is enough, let him go while you hold him and tell him you love him. I am sooooo sorry, hope I'm wrong. Best of luck to you and your wonderful kitty.