It sounds a lot like innatentive ADHD to me. I have innatentive ADHD, but it wasn't diagnosed until my 40s.
I was a bright child and expected to go far. I messed up my A Levels through playing computer games late into the night, instead of revising. I still got into a university through clearing, but I just couldn't cope with the unstructured nature of it and I didn't complete the first year.
My mum just let me get on with it. But, that didn't cure my ADHD!
While he does need to learn to adult, please don't just leave him to sink of swim, he may well just sink.
FWIW, here's what happened to me next. I drifted, signed on, found a boyfriend who was unemployed and spent a lot of time smoking weed with him and my friends.
Eventually I got a job in a pub which I loved as I was very sociable, but I drifted in and out of that kind of job. It wasnt until my mid 20s that I started working in offices and building a career. My undiagnosed ADHD still made things tough - my self-organisation was all over the place and I was often late. But I was bright and keen, and finally able to do something that was using my brain so I was promoted quickly.
I eventually went back to university in my mid 30s and got a first. While I loved being at university as a mature student, I did feel sad for my younger self for missing out on the experience. Since then I've worked hard and my colleagues now would be surprised to learn of how I used to be! (My ADHD runs wild at home instead now. Sigh).
I think it's a shame I didn't get a little more help with direction - I was just left to get on with it and although I did eventually come round to building a career, it took several years to come to that decision by myself and I lived for many years after that with very little income.
I would start the process of diagnosis, if he's open to it. Please do help him with this. It's really helped me in understanding myself, and if the meds work for him, it might actually be the answer he needs. The school aren't necessarily experts! The SENCO at ours was awful when we started the process of diagnosis for DS (ASD) and if we'd waited for the school to take a lead it would never have happened.
Otherwise, I think it's possibly a case of meeting him where he is. Yes, something needs to change, but maybe it's a move from telling him what to do, to supporting at more of a distance.
Good luck OP, this stuff isn't easy!