Long answer @Thebabycheeses78 but hoping the tips/suggestions are what you are looking for.
I think your expectations -and probably your son’s- are unrealistic. At 15 most AuDHDs have an emotional and organisational age of 3-5 years younger, even if they are academically high functioning. My DD is 20, now at uni but we have to oversee stuff, eg when packing for uni or holidays.
I break it down into smaller tasks, ranked by by priorities:
Have you packed your toilet bag?
Did you remember to put your meds in it?
I will then ask her to go pop it in her case.
Do you have your underwear and night things? Great. Let’s put those in.
Where are your clothes and shoes? Shall we put those in next?
Do you have your tech/charger? Which text books do you need this term?
I often do actually write a post-it (or prompt her to): toiletries, meds, tech, underwear, shoes, clothes [might write: 2 x trousers, 4 x Tshirts, +hoody] after a previous chat where we’ve discussed what she will need.
The post-it note lists goes in the case as it can be reused at the other end.
I also do reminders. ‘Can you make sure you are packed by 7?’ [I know we are leaving at 8, but this leaves time to help if needed]; at 7, if she is not ready, I ask her if she can be ready in 15mins. I will also ask ‘if she would like some help’ at this point.
Tbh, this was my routine at 15/16 - for both my AuDHD kids (DS 17, DD 20), but after several years of prepping a packing list with them and setting a time line, they really have managed for the last few years without any help. The only thing I now routinely check is whether they have their meds.
It sounds exhausting, but it becomes easier - try to get him involved in writing the checklist so he is empowered, rather than you imposing (if he has any ODD/PDA elements to his AuDHD, it will trigger a push back, so it is aways best to frame questions openly - eg rather than ‘pack your shoes’, ask how many pairs of shoes do you think you should pack? Do you want to put your Vans in? Etc). Am sure there are other tips, too. Hopefully other PPs can embellish.