Perhaps you are over thinking this.
I am financially embarrassed so I tend to think of the simpler things first.
My first thought is to trim his back nails. Perhaps you have done so but it isn't clear in your post. Keeping all the nails trimmed is the easiest and cheapest way to substantially reduce self inflicted scratching injuries. If necessary wrap him in a towel to keep him from injuring you while you trim.
My second thought is the water dish. You mentioned changing the food dish but didn't mention the water dish. When my big male started having issues, with what I simply diagnosed as cat acne, he scratched the heck out of his chin. While I provided fresh water several times a day I was not cleaning the water dish often enough. I started using hot water and scrubbing the water dish out daily, especially around the rim where he would rub his face on it. His problem cleared up after a few weeks.
Third: Fleas. No one likes it when I mention fleas. Their initial reaction is to absolutely deny that their cat has fleas. Well it doesn't take a lot of fleas if a cat is flea sensitive. I have one cat that a single flea bite will set her off scratching for days or even weeks. Even in the purest most flea diligent household a cat is going to get an occasional flea or two. So it should't hurt to try one of those once a Month flea treatments. I currently like Advantage II since it also seems to make their skin less dry. I was using Frontline but after a few years it suddenly stopped working (not sure if fleas got resistant or I just got a bad batch) and I ended up with a house full of fleas. I'm guessing that most of the once a Month things work about the same.
Forth is the FIV thing. Have you had Jarvis re-tested? I read that the tests for FIV test for FIV antibodies and that it is easy to get a false positive. A cat less that 6 months old can test positive from antibodies it received from its mothers milk. And a cat that has ever been vaccinated for FIV will test positive for FIV. You said that you got Jarvis last fall and that he is 18 months old. Depending on when you had him tested he could have been under 6 Months old and tested positive because of it. He could also have been vaccinated by someone and tested positive because of it. Ask your Vet. about it. I'm guessing that as long as your other cats have been vaccinated that they are fine to be with Jarvis.
Finally is the socializing. When I took in Motly, my big male Maine Coon cat, none of my 3 girl cats liked him. He was big, clumsy, extremely shy, basically a scaredy-cat. He spent the first 6 Months under my couch most of the time. Eventually he made friends with the 3 other cats. He was inside for years before any of my friends even saw him and even now he will only come out for a few select people. I would let Jarvis run with the others but make sure he has a few places to hide. Motly likes to hide under things, couch, bed, basically any type of low furniture. Others like to perch up high like shelves and book cases. On top of warm computers and DVD recorders are real popular.
PS. Trim everyones nails so they don't hurt each other while Jarvis is being socialized.