How does he feel about his diagnosis?
Mine was transformative for me, but I was older. I pushed for DS1 to get a diagnosis, which he did at age 10, but he hasn't been as enthused as I was. We've had lots of interesting chats but whereas I have obsessed over ADHD pages and forums and books and podcasts, he's been mildly interested and that's about it really.
Honestly, one thing I would 100% encourage is for him to follow any dreams and ideas that he has. Rather than assuming or worrying that he won't be able to do it - try to support him in ways which he can. Looking at alternative options is fine, but only if they are presented as an option rather than trying to persuade him away from A Levels. In hindsight, I would have done much better continuing at school in A Levels, because the school environment is structured, and that worked very well for me. After GCSEs (where I had been predicted mostly As with a few Bs and Cs, but ended up getting mostly Bs and Cs with a couple of As) I went to college and did a BTEC in a more vocational (creative) subject and just floundered, because the course was so very open and much more grown up in terms of basically giving us freedom to study and choose whatever. It was MUCH too open for me, I couldn't cope with the self-motivation, time management and organisation required. I didn't know I had ADHD then, it wasn't routinely diagnosed in non-hyperactive bright teenagers at that time.
I struggle to wake up in the mornings without medication. So does DS1 (it's really the main way his ADHD affects him at the moment - he does not want to take medication.) I can explain how this feels for me if it would be helpful? Both DS and I find that a QR code alarm is very helpful - this is an app where you have to scan a barcode, we use the barcode of the soap dispenser in the bathroom or a brand of shampoo/toothpaste which we always buy.
DS also finds it helpful that DH installed a smart bulb in his bedroom light and it's programmed to come on in the morning. A light alarm clock would probably serve the same purpose. For some people with ADHD, there is a lack of the correct sleep-cycle hormones or they occur later in the day, so trying to wake up at 7am feels like trying to wake up at 5am. I do a lot better if I can wake up after 8 and the best if I can sleep (or at least snooze) until 9ish. Trying to wake up too fast is also a disaster.
Combine that with the fact that teenage circadian rhythms are often later-shifted as well - you can see the issue.
If he struggles to fall asleep at night, talk to the GP as they may have some suggestions (although I think they will probably suggest sleep hygeine first).
If you have access to one it might be worth speaking to a neurologist about medication/tics. I believe that it's basically similar to other side effects in that because the medication is short acting, if he stops taking it then they should go back to normal again, so it ought to be possible to try it and see. But also that they may be worse initially, and then calm down within a couple of weeks of being on the medication but you might get resurgence of tics as it's wearing off. They may want to titrate (wean him up) more slowly than usual. This is normally done slowly anyway, but it might be better to do it even more slowly?