ADHD can be co-occurring with a condition called Delayed Sleep Phase Disorder which is horrendous as you constantly feel jetlagged. It can be diagnosed with a sleep study but I don't think there's any treatment for it apart from melatonin and stimulant medication, so the NHS might not be keen to investigate this.
But also, ADHD can make it hard to stick to routines and be aware of your own body signals. The transition into and out of bed are both really hard, and if you add that to ordinary life stuff which makes it hard to stick to a routine like - adolescence (no idea how long the nocturnal sleep rhythm thing lasts for), internet scrolling/gaming, streaming TV, having no regular routine - this just all makes it really tricky and there can also be things which don't work easily like trying to mesh his routine with other people's if you need to time things like showers and using the bathroom not to use up all the hot water or disturb people's sleep etc. And then poor sleep hygiene in general can feed worse sleep because if he's sleeping more than 8/9 hours in order to "catch up" that can make you groggy etc and if he's missing a lot of "typical" daytime awake hours then he might struggle to socialise in real life or see much daylight.
The right medication can help but it can take time to get the right medication. Until a couple of weeks ago, I would have said I was on the right medication, but I still struggled to wake up in the mornings because I just felt like a total zombie like I wasn't conscious yet for ages. I changed doctor and she suggested trying one dose higher and it's really helped - I now feel alert within a few minutes of waking up as long as it's a normal time, and I don't need to take the medication before I get out of bed and then set an alarm for 30 minutes later to actually wake up. I can't explain it because I would have assumed that the medication would need to be IN my system in order for me to feel alert in the mornings, but it seems maybe it's just helping me feel less tired in general and that is having a knock on effect making mornings easier.
If I can give you some advice, I'd try to separate out any "moral" ideas you have about sleep e.g. it's lazy to sleep past 9am, it's unhealthy to have an unusual sleeping pattern (or whatever) and let him try to figure out what sleep pattern works for him without giving him guilt about it. If I could follow my body clock perfectly, I'd sleep roughly 2am-10am - which is not that weird - it's still 8 hours of sleep, yes I'd miss the early morning bit but not every job has to start at 8 or 9am. Some people are shift workers and sleep during the day, working at night or shift their sleeping pattern based on whether they are working nights or days. If he is naturally leaning towards an unusual sleep pattern, but he feels pressure to try to stick to a "normal" one then he might be shooting himself in the foot and not getting the best of either approach.
You say he has a job but is this a 9-5 type job at a location he has to get to, or is it flexible WFH, different times on different days or not every day?
In order to make plans it might help to text him, then you don't need to be in the same place at the same time. If he knows that he has agreed to help his grandparents, he might have already arranged a time? If not I would try to wake him up around 10/midday and remind him that he promised to help them. What's happened before when he's done things like this?
How do you approach mealtimes - do you tend to eat together or does he do his own thing?
I find this author quite good, the book cover looks a bit naff but her advice is pretty solid. www.amazon.co.uk/HOW-DAMAGE-YOUR-ADHD-ADOLESCENT/dp/1739958837/