My AuDHD son is also 13 and in yr8 in a mainstream state school, no EHCP though. I took him to see a paediatrician at 18 months as I suspected he was autistic and preschool suspected too and got him seen by the LEA Ed Psych, but nothing came of it - it wasn't until he was 8 that he was finally assessed following a letter from his year 3 teacher to the GP.
We applied for an EHCNA when he was in year 6, just after his ADHD diagnosis, but our LEA turns down the majority of requests they receive and I didn't have the energy to fight it as I was also appealing his secondary school place at thst time (lost that too!). We had planned to send him to a small private school to limit the exposure to loud, busy classrooms/corridors - but everywhere we applied to rejected him - I must say I had no sympathy when Labour brought in VAT on private school fees having been treated the way we were (a whole other story).
Fortunately the school my son was given is really inclusive and collaborative. They have agreed to us flexischooling our son, so he spents one day a week at home doing 'home ed' (basically pursuing his own pursuits like coding/nature walks) and then 4 days in school. He still really struggles with school, always begging me not to send him, and telling me he doesn't learn anything at school. His academics are still high at the moment, part of me can't help wonder if it is just that the average level at the school is lower than other schools, but I do believe he is doing as well as he is currently because he has the day at home, and his exposure to the school environment is reduced. We are fortunate that my husband doesn't work, so there is someone at home with him on his flexiday.
I do worry though, as to how long this status quo can last. School want him to reduce to half a day of flexi next year and then full time school from yr 10 so that he won't miss any of his GCSE syllabus, but I don't know whether he will be up to that. I also worry that once the curriculum heats up in year 10 whether he will be able to cope with the pace. Also he isn't medicated for the ADHD and I know he struggles to focus, but CAMHS are about to discharge him as he won't take the medication, so if he needs it for GCSE we may be out of luck, unless we can find a private doctor to prescribe it.
I don't have any advice I'm afraid, but I fear we will be in a similar position at some stage. I feel the flexischooling is helping, not sure if that is a feasible option for your son given you both work ft and many schools are very reluctant to agree to it - logistically it can be difficult. What saddens me, and I suspect it does you, from what you've said, is the feeling of wasted potential. School advertised an afterschool maths club recently, and it looked fantastic, but my son doesn't want to stay in school any longer than he has to. Also, they're running a school trip abroad which also looks right up his street - but he doesn't want to go away with people from school - it also clashes with his flexiday and the weekend and he doesn't want to miss his time at home.
I do think that our ND children will come into their own over time, just perhaps not following the traditional mould.