ADHD can put you in such a vicious circle, and although he was clearly in the wrong I really feel for him. I went to University and in my first year I dropped out and told no one. The reason was because I knew the sacrifices others were making to allow me to go. I also knew there were high expectations, even if they hadn't explicitly been set.
When I eventually managed to go back, I unreasonably put myself it such stressful situations by not handing in work on time or leaving things to the last minute.
You maybe already know but with ADHD we need external pressure to do things like uni work such as deadlines but the problem is that it's so close to the deadline we're able to start that sometimes it's simply not achievable.
My best advice would be to tell him how much you love him, tell him you understand that he's clearly been in such a difficult place and he's not alone. I would look up ADHD burnout and actual rest and I would encourage him to focus on self care (proper self care not just lying on the sofa doom scrolling as we adhders tend to do because we might be physically resting but we're actually mentally listing all the things you should be doing).
Then I would (if you can afford it) give him time to recover, maybe mentally set a goal of 2 weeks or a month where you don't actually put any pressure for future plans you just focus on him getting to a better mental state.
Before I got my diagnosis I got to the point I was severely unwell and just doing a month of actual rest and speaking about how I was feeling honestly worked better than my antidepressants because burnout looks like depression but it's not.
Then after this I would encourage him to look at volunteering or princes trust. There are loads of apprenticeships out there also. What I would also advise is to not position it as needing a life long career, us ADHDers are not meant to do one job forever - we work well moving roles even within companies. Creative rolls are great too, can he be outside? Would he consider physical work?
Also get him to look up the podcast by Alex Partridge - he started Lad Bible and has a fantastic podcast about having ADHD. The Adult ADHD toolkit book is great as well.
Lastly, it's ok to grieve the life you thought he'd have and be upset with where he is. ADHD is a disability, as much as some people like to call out only the positives there's a reason only 15% of ND people are able to work full time. Helping him to understand his ADHD better wi absolutely help him get to where he wants to be
Sorry for the brain dump but I hope this helps and I hope it gives you hope that this can get better ❤️