Ok, well if the biting and screaming aren't new then I wouldn't think it's a vet issue if he has been seen, evaluated, and treated for another issue while this has been going on.
Good; you know the REASON for the biting. Now you need to teach him that it doesn't work. My suggested method is that when he bites you do not react by yelling, flailing, or any other "fun" reaction that might amuse him. Instead, instantly drop him on the ground and walk away. When he comes back to you you can pick him up again, but the dropping needs to be INSTANT. It would also be wise to avoid holding him and things he wants at the same time.
Ringnecks do well with pellets as a main diet but they really need veggies and fruits as well. Particularly orange, red, and yellow ones as well as leafy greens. I would incorporate those right away which will greatly add to his enrichment. I also suggest sprouts when possible. Not only are these important additions to the diet but foraging toys or even just knowing that baby carrots, apple slices, etc can be found, can keep a nosy piggy (I mean ringie!) busy and happy for much longer than a toy that doesn't taste as good

using up that mental energy relieves enormous amounts of their frustration.
Below, Irns enjoying fresh food and foraging toys, as well as a link to my "how to" on sprouting.
It sounds like your screaming problem is two fold; he is bored and he feels ostracized from the flock.
He really should have as many "play stations" as possible so that wherever you are he can be close by AND ENTERTAINED at the same time. Expecting any parrot but especially an IRN to "sit still" for any length of time is a recipe for disaster. They just aren't designed for it. They are too smart and too curious. In order to fit in well to a human family and not destroy their home, they need lots of different toys because you are right; nothing holds their attention for long. We have little make-shift play areas all over the house. We used to have a big tree with lots of additions but ultimately it proved too large for our tiny house but I'll show you a picture of it. My favorite "play station" is actually a toy box filled with foot toys and random things like bottle caps. It is easy to replenish with new things so they are always hunting for something exciting. It's also great because it's easy to move and easy to store. My birds love them! You can even toss a hand full of see in the. Bottom and poof! Foraging toy!
Another note; Irns seem to love stuffed animals. Don't put them in the cage, but tying one to a perch can mean hours of entertainment (in short busts). Be sure to replace them as they become worn.
Anything can become a play stand. Kaleo loves this mineral block that we tied to this lamp. So easy!
this is one of those kiddy play blocks with non toxic paint. They love it. These two are Madison and Hamilton.
Nohea also loves the lamp; she loves to turn it on and off. But she mostly loves it because it is right by couch where we spend our evenings. She wants to be close to us.
Saeed and Chin Ho demonstrate their love of stuffed animals.
Nohea and Hammie show off their toy box.
Our big tree used to be right next to my desk (I work from home) and was perfect with lots of toys and food stations to keep busy beans and brains occupied.
Sent from my iPhone using Tapatalk