The neurological disorder that Fran mentioned was Prader-Willi. If you google it you will get some interesting information. If he is eating non-food items, it could be pica. That can come on due to nutritional deficiencies or for other reasons and is another neurological thing (pretty sure it is neurological). Though now it seems there is also a Binge Eating Disorder that they are treating with medication, so that may also be a thing to consider. I cannot recall which medication it is, I saw a commercial for it.
As for getting them off of the computer, that is always a chore. It was never easy with my kids either. Is there a way to have a talk when he is not on the computer to explain that when he kicks and screams and says awful things, it makes you want to say that he cannot be on the computer for an extended amount of time, say a week? Tell him that you don't want to do that, but when he goes into a rage when his computer time ends, he scares you. It isn't fair or right to frighten the people in your home. How would he feel if you had to stop using to computer to go fix dinner and you yelled and screamed at him over it? Can you take a video of him throwing his fit and then sit down and show it to him? Don't sit down during the fit. Wait until the next day, at a calm time, and then tell him you need to talk to him. Ask him what he thinks about how he behaved? Was it fair to the rest of the family? Was it responsible? Did it show self control and maturity? Could he hurt someone? How would he feel if he hurt someone? What if he broke the computer during his fit? Certainly if it got broken during a fit and got replaced, the person who threw the fit might not get to use the new comptuer much, would they? They couldn't be trusted to be safe with it.
I know you are using reminders and timers and things, but are they getting his attention? Is he actually aware of them? It is my guess that he isn't even aware of the timers or reminders. I know that may be hard to believe. My kids could even reply to me that they heard and understood, but often they had no idea what they heard and understood. I had to make them stop what they were doing and LOOK at me, and actually engage with me before I reminded them. Otherwise I was just like the teacher in the Charlie Brown cartoons "Wah Wah Wah" in the background. That is why I came up with using chocolate pudding, chocolate chip cookie dough, etc... with my family. That was currency that worked for them. It might work with your son to. I bet he could hear you talk about fixing everyone a slice of chocolate cake even if he was on the computer. Maybe not. There may be something else that is his "currency", the key that will gain his attention.
I realize you may not want to use food. That is fine. When you remind him, go to him and gently make sure his face is looking at you, that he is looking you in the eyes when you remind him. It is one way to make sure he hears you. Once you have him looking into your eyes, then you speak. Not until then. That is a trick I learned from a special education teacher I greatly admired. She would know a student was absorbed in something and wouldn't hear a reminder or directions to do something. Rather than yammer over and over, she would turn his head (gently) to face her, get onto his eye level, and when he was looking into her eyes, she would say what she needed to. Then they could go back to their game. It was astoundingly effective. Maybe it will help you.
