My son and I have adhd.
Ask the school for support for him first of all. You don't need a diagnosis for a school to support properly.
Ask for the school to refer or ask your GP to refer to Camhs. To be honest it's up to a 4 year waitlist for assessment. If you're looking to medicate you could be another year on top of that. Youcould go private and I believe England has something called right to choose. Adhd 360, Mypace are used a lot.
There are plenty reputable companies out there qualified to diagnose.
Ask your GP if they will share care should you choose medication (if it's offered)
I'm in Scotland and my GP shares care so we are seen privately for med reviews and the GP prescribes our meds for us.
School and LA on board with private diagnosis - schools cannot ignore a private diagnosis so don't let them tell your that.
I knew when my son was 2 that he had adhd, he was so bouncy, ran all the time, if he woke up he would spring out of bed, no sense of danger whatsoever, couldn't sit to eat, watch a film, he was like the energiser bunny on steroids. He just wouldn't stop, getting him to sleep was a nightmare, blurted out, couldn't wait his turn, aggressive outbursts and he was obsessed with his iPad. He also has autism. Difficulties with friendships (now resolved and absolutely fine now he's 10)
I was slightly different, I was diagnosed with ADD at 14 (now rediagnosed with adhd combined) my hyperactivity wasn't running, it was speed reading, talking fast, oversharing information, my emotional regulation was shite. I struggled with friendships, blurted out answers, had extreme reactions to rejection (being picked last, losing at something) There's so much more but that's just a couple of things.
I hope you get the support he needs. Start with the school first and foremost. Don't worry about next year, start working with the school to plan for next year so he can settle in. He will be needing fidget tools, ear defenders, hot desk, movement breaks, brain breaks, short clear instructions, prompts, reminders, now and next board, etc. Ask the school to help with friendships. Small groups, rotated round. If he's having difficulty and becoming frustrated with writing, ask for a laptop instead. Short writing work should be encouraged but not at the sake of his well being. My son doesn't write anymore and hasn't for the last 2 years. He works on a smart board and has a laptop in class and he has a laptop at home too.