I agree with the ADHD assessment - alongside putting in place some strategies to organise himself pending the outcome. Even if he gets a diagnosis, it still needs this. I really strongly recommend that he sets himself a hard pre-deadline deadline for each piece of work, probably 1-2 days before it's actually due in, so that he has time to go over the complete draft, and can then turn it in calmly the night before or morning that it is due. If you are prone to deadline-itis, you have to get used to moving the deadline yourself or will never get through a dissertation, for example (for that, I recommend 3 weeks before for a complete first draft). Move the panic all-nighter stage forward, and allow then some time to actually perfect the work. My attitude was always that being a student was my job - it was a minimum of a 9-5 and the lectures and taught sessions were meetings that I would have to attend in the job, and that the 3 years were about building the skills for the real world.
He also really needs to deal with the getting up alarm thing over the holidays - it can be really dangerous as well as very annoying to everyone else in halls. I spent the first two years of uni living with someone who slept through alarms, drove us all insane, it got to the point when we would be banging on her door in tears telling her to turn the bloody thing off after it had been going off every 5 minutes for 4+ hours when we were trying to study. She became so used to be comatose during alarms, she then started failing to wake up for the fire alarm, so we used to have to bang and bang and bang on the door until she came to. It got to the point where she had to leave her door unlocked at night so we could go in and physically shake her awake.... which was just as well as one of the fires was a real one. But again, a diagnosis and medication won't resolve this, needs to deal with it.
The not eating won't be helping with the getting up, probably running on empty. Does he have a kettle in his room? If so, things can cook with just a kettle (noodles, cous cous, quick cook pasta, and a pyrex dish where can leave to stew in the boiling water) were things I found really helpful when I couldn't prise myself away to cook properly, or couldn't physically get in the kitchens to cook.
Include food as part of the routine that you will no doubt be working on him to put together over the holidays. So Monday night is pasta. Tuesday is Taco Tuesday at the student union and I will budget £5 per week for that, etc etc. Grab and go breakfasts - cereal bars, bananas, protein shakes. Not ideal but something with some calories and if you pick well some nutrition.
Set time limiters on the games. No more than x number of hours at a time, and nothing will work after midnight. Yes you can get round them, but it sounds like its the reminder to either do something productive or go to bed that's the issue.