Does he have a relationship with a psychiatrist or therapist? Discussing this with someone other than you is likely to go better.

My two college sons (twins) have had the same experience you’re describing. Great habits when they left for school and now (one more than the other) they’re falling behind, struggling to keep up and there’s been drugs and alcohol too. It’s been rough. We thought after break they could get on a better track as they promised. We said something needs to change or mom and dad are coming to live with you! We were only half kidding. 😂 Now they’ve been working with a coach (one also has a therapist since high school) and things have been improving some. The coach has been helping with balance & habits - sleep routines, homework/study plan etc and if they party, she’s working on what choices they make around it. They are more open with her than with me and my husband and they respond better if it comes from the coach rather from us. She meets with them
virtually and she’s very good. So far it’s been a working quite well. Hope that’s helpful.
Good luck!m

DC did great the last few years of H.S. - had developed really good habits around sleep, social media, meds & exercise, figured out good coping strategies for time management, getting assignments in, etc., and was managing everything mostly independently. Then went off to college and wanted to be social (great!), started partying (alcohol, pot, nicotine, likely other drugs), developed inconsistent sleep habits, and rarely takes their meds. Grades are reasonably good (though could be better with some discipline - there’s some class skipping & forgetting assignments) and they don’t seem depressed, but their motivation (including to job search for internships) and emotional regulation have devolved (outbursts, etc.) - whether from the substances, poor sleep/lack of exercise, or lack of meds, we’re not sure. We’re not subsidizing the partying (they have a campus job), and aren’t completely against it, but are concerned about the direction it’s going. We’ve discussed the risk of substance abuse for people with ADHD, but they seem to feel pretty invincible at this point.

Has anyone had similar experiences and been able to steer their ADHD kid back to a better course?