How old is Jinx?
I went to a feline only vet and it made a world of difference with an issue other than yours, but I ditto silentmeowlook on that as well as seeing a vet sooner than later.I'm going to say "third" on the fact that he needs to be seen by a veterinarian, sooner than later.
We have also used the plastic garden pokey things from the dollar store that are meant to keep cats out of the flower beds. They fit perfectly on couch cushions etc and are easy to toss out of the way when you are using the furniture, also still soft enough that it’s not going to hurt anyone if they accidentally sit on it, or jump up there. Worked perfectly for training which furniture was his and which was not.You are going to have to make the couch unwelcome to him.