@charliebear78 is he year 9? My ds is year 10 now (14 in the summer) but he was diagnosed with ADHD at the end of year 8 and year 9 was awful (year 8 hadn't been great!) - I was so glad it was cut short. To be fair lockdown was much better.
A lot of what you say sounds similar - he hated his diagnosis, didn't care about homework, was disorganised, always getting in trouble for either silliness/disruption and late/lack of homework. It was a real challenge!
I'm not sure what has happened except for maturity... but this year so far has been better. I still check his homework app every night and remind him of what needs to be done, but he doesn't fight me over it. To be honest, I'm not sure he spends long enough on homework, but he's doing it.. and in addition, he's working in class, which is the bigger priority for me. He's even got a few merits! I figure that if he concentrates in class, that's the main thing - and it's pretty exhausting so he probably doesn't have much left for homework.
One thing that did help I think was him starting to own the diagnosis and not be ashamed of it. He sometimes posts funny things about ADHD on snapchat, poking fun at himself, but we've also got him to see the good side too - for him, this is that when he gets into something he can really focus and get very good very quickly. Also, we've acknowledged together that he can't focus on work in the evening for long, so we try to encourage short sharp bursts. He's gotten very into physical exercise too, which I think helps. He can expend all his energy on doing a work out and then he can concentrate for a bit.
I think with ds too, he would get into a cycle of behaviour points/ getting in trouble at school and he'd then be sullen and fed up and get in trouble at home too. It became a vicious circle that made him feel very low about himself. So I'm keen to help him not to get in trouble wherever possible - although obviously, a lot is down to him.
But most of all, I think he's just grown up a bit. He's started thinking about what he might want to do next and knows he needs to get his GCSEs.