This is something I think about a lot. When was the last time that someone who was addicted was helped by someone OUTSIDE that person preventing access to their addiction of choice? It's not possible. I mean, it might help in that it would allow for other passions to take root if he weren't 24/7 on one passion. But as for self-discipline, no one outside the person can develop it for them. They can't tell them when their body feels full of food and it's time to stop eating; they can't tell them what it feels like to get dizzy from too much alcohol and maybe learn to hate that loss of focus & control, they can't tell them when their eyes are bugging out from too much screen time. etc etc.....what I'm saying is the "self-" part of self-discipline really does mean that it's something they need to develop. And that is why I have never fully micro-managed such things, as much as it tempts me.
They say that (law of attraction-wise) that you get what you think about the most. You will attract what you most fear. Sheesh. That alone is an argument for letting up.
And another thought is, if it IS addictive and what I'm seeing IS addiction, then that ship has sailed. I have nothing to lose by letting it continue to see if (a) it will abate naturally & maybe it wasn't addiction or (b) if help or intervention really ends up being needed. Because seriously, the short-term effects are mostly on us, his parents. As for him, he's fine with it. He's having a blast. The only negative to HIM is US. In other words WE ask things of him and he moans or complains about it. It's we who have problems with worrying about his future because he doesn't do X,Y or Z (we parents SO need to be deschooled!)
Maybe I worry too much. So he doesn't do chores or study his ABCs. I am the one with The Waltons in my head; maybe I'm the one with the problem :-)
The reason I don't worry about my ds's extensive computer use is that he is generally willing to get off the computer to do something else fun or interesting. No, he won't choose to go help me garden, or ride his bike around the block, and he might be reluctant to go on a family walk, but he's happy to meet a good friend for a nature hike or go to a cool museum or something. He has been using the computer all along so getting to use a computer is a given (he has his own). The kids do need to learn how to moderate their use and balance their lives. Since ds has been using the computer all along, we've been guiding him with that all along, mostly focusing on getting some outside time and some exercise.
And I've head the same experience as you with my own use. The internet is great fun but eventually aspects of it get old and boring and I'm less interested. Then something else comes along, like Ravelry, then Pinterest, and my interest in the computer goes up for a while. Until I start to notice how repetitive it all is. I've had the same experience with TV series. I'll lose interest because the characters keep getting into the same predictable stupid situations. Because my ds has gamed for years, I've seen his interest in many computer games wane. A few are really good games and he still revisits them. Minecraft is new for him, too, and we've had many walks and nature hikes where all he wants to do is discuss it. It seems to not be the main topic as often anymore. But he still plays it daily in rotation with other games.
If your ds gets accepted to the Sudbury school, I'd say resolve not to worry until he settles in there.