IME you have to let them fail a bit and experience the natural/real world consequence - it's horrible but the problem is if you do it all for them forever and then suddenly let it drop they don't develop the coping mechanisms and skills on their own.
That is not to say that you shouldn't support at all. Sometimes it's not the right time to give them the space to fail, because that would counteract progress with something else, like school anxiety for example.
Is he on medication? For me personally (I also have ADHD) mornings stopped being such a massive nightmare once I found the right medication. Although I only take it when I wake up it seems that either it reduces general life exhaustion so I'm sleeping better, or there is enough left in my system from the previous day that I can actually wake up in a fairly normal time frame rather than being a total zombie until about 10am regardless of how much sleep I'd had or continually falling back to sleep until the afternoon if uninterrupted. I seem to really struggle to get going in the mornings naturally, and DS1 does as well.
Shouting/urgency tends to work because it "wakes up" the central nervous system which seems a bit slow to come online in the mornings. You/he could look for and experiment with other ideas for how to achieve that. For example, DS2 (nearly 7) is in OT at the moment and they have looked at energy levels with him in terms of self regulation. His nervous system gets really easily activated which results in aggression because it's like he's perceiving everything as an emergency which needs the strongest possible response right now, it's like he has too much urgency. So for him we are looking at calming activities to signal to his CNS that there is in fact not an emergency, it's just your brother being annoying (or whatever). But on the back of that sheet there are suggestions for how to wake up or activate this urgency/alertness level, and it's things like bright lights, upbeat music, sense of urgency, movement, cold water, enthusiasm from the adults' voice/body language (You know... all the things you just LOVE to do early in the morning 😱)
Anyway... not really my preferred solution. But it might be something you can do in a more gentle way. What kinds of things get him all enthusiastic and wired up? Could you incorporate those into the morning routine? The problem of course is finding something stimulating enough to engage him but that he won't get distracted with.
Simulating the urgency but handing it over to him might be a way to do it for example setting multiple alarms or reminders, esp on some kind of voice assistant like Google/Alexa, reminding him of what stage he should be in. Sort of mini-deadlines for each subtask, like OK it's 7:00, the joke/fact of the day is X (I'm sure this is a thing you can set those assistants to do), time to get wiggling your toes and stretch under the duvet, the weather is XYZ.
Now it's 7:15, you should be heading into the bathroom to brush your teeth/starting to get dressed. If you can get downstairs and dressed before 7:20 you can get 5 points (maybe this could also be randomised).
Tying routine tasks to a reward that can build up over time can help too especially if there's something he really wants.