Glad to hear he did not become violent when his devices were taken away!! I'm very surprised he is not prepared to go to school in exchange for device time, I hope that could change in a few days? How long has it taken him to come out of this in the past?
I really hope you can work out a way of getting him out of the house, both to go to school and to see a doctor. I have no idea how to deal with a 6ft teenager!
If he remains unwilling to go to school, can you explain to the school about his mental health problems and possibly negotiate he still attends school later in the year when he is able to but could repeat the year if he does not do well enough?
Slightly wacky suggestion, but does the school have flexi boarding? It sounds like you've lost authority with him as he has no intention of doing what you ask? I wondered if he did say, 3 - 4 nights of boarding a week then he would be forced to behave and to work and would have no access to gaming and would have a full and busy social life? Or even some kind of residential activity camp which you can sell to him as a 'holiday'? Having to fit in with a prescribed routine and having something to do all the time might bring him out of his current state?
If you don't want him to board, I agree with others about giving him lots of hobbies such as the Cadets / RAF stuff. If Dopamine is implicated in addiction, I imagine excerise would help? (Lots of research on Dopamine and excercise in relation to ADHD.)
I'd also have thought a lot of the advice relating to gambling addiction could be relevant? I imagine finding a psych specialising in gambling in the UK is a more realistic prospect than finding a gaming addiction psych?