Hi so firstly, congratumisserations.
It's liberating getting answers but it opens up a new door to grief of a life you expect them to have vs. the barriers they face.
Yes, absolutely apply for an EHCP. You can apply yourself through the local LEA website, and is often quicker to get the process started. A diagnosis isn't necessary for an EHCP as it is needs based but now you've got evidence of a diagnosis it can absolutely help.
You need to decide whether you want your son to stay in mainstream, or whether you think a specialist provision would be better.
If it's specialist then you also need to look at both primary and secondary or see if you have any local through schools.
Our LEA do a needs assessment with an Ed Psych evaluation as part of the EHCP process to determine primary need. This can be social communication and interaction, social emotional and mental health, physical, medical etc. Not all specialist provisions cater to all needs.
You've also got to remember that it's hard enough to get an EHCP so in order to get into specialist you need to evidence why mainstream with interventions just won't cut it. Usually the school will do a few cycles of Assess Plan Do reviews to help support you, if you choose specialist.
If you're happy with mainstream and just feel like he needs additional support in mainstream, that's wonderful too, but the school should still do the assess plan do reviews, and this can help identify areas of weakness where reasonable adjustments such as longer exam time, 121 support, therapy interventions etc can be put forward to be put on his EHCP.
If he is a school avoider (EBSA) then you may be entitled to education other than at school (EOTAS), which is usually online however it doesn't strictly have to be academic education at all and can be education that supports interests. This is usually a last resort for LEAs though, but it is still a request you are entitled to make if you feel like your hands are being tied and pushed towards involuntarily elective home education, as it keeps the LEA responsible for providing your child an education.
You really have to have sharp elbows as a SEN parent, and be aware that schools however well meaning are not clued up on neurodivergent affirming processes.
You should also look to see if you qualify for any DLA. I would seek advice on this, because it's a minefield and the way you phrase things can make or break the application, but also when the past 9 years has been your normal, it's hard to identify areas of need that you could qualify for.
I would also see if you can get a referral to occupational health for a sensory diet assessment, this will help everybody understand your sons needs, what he avoids and what he seeks.
For example some people thrive off of loud background noise but are completely overwhelmed by sudden loud noises, unless they are the ones making them. Some kids bounce off the sofa and climb the walls, and constantly rugby tackle or look for rough and tumble play to meet their proprioceptive needs, but light touch that most would ignore can be extremely painful. For others the sound of eating, drinking, chewing, burping, etc can cause such a rage known as misophonia, and others might not even pick up on it, similarly some might get extremely agitated when things move too fast known as misokinesia.
As he grows older you might want to consider a care assessment through the care act, this can grant access to supported living, or a personal assistant, access to priority council housing, cleaning services, etc. I have just had one done for myself at 30, and have been positively overwhelmed by the support they can offer.
I'm sorry for the huge wall of text, it can be completely overwhelming but if you've got any specific questions I am happy for you to send me a DM.
I am an autistic parent of an autistic child also going through the EHCP process, but quite a bit further down the line, so happy to help where I can.