Definitely apply for EHCP yourself. ASD we would be looking at SALT, especially small group classes to help with learning social skills.
ADHD we would be looking at lower distraction learning like own work station etc.
The behaviour could be down to the increased frustration from un-met SEN - seen plenty of this over the years of helping folk with cases, and of course the dv could have triggered it - but the two things in combination is a lot for poor old dc. The dv is in the past, the continued failure to meet his SEN is what you can do something about, so well done you for getting going on it.
NAS have trained helpline volunteers - you leave a message and they get back to you 0808 800 4102 - they will help you file a request for an EHCP and progress through that. IPSEA does something similar but is not ASD specific.
PRUs are usually used for kids who have behaviour issues as in disruptive behaviour, and is an alternative to exclusion, but that said, they can have a much higher staff to pupil ratio and lots of praise for target achieved so might be a good backstop and also help you get some more info on what dc should be getting for his ASD/ADHD.
Any chance you can get indie EP and SALT? If you are on Legal Aid and start an appeal then you can get legal aid to pay.
If you are in Kent then the Bedgebury Foundation may contribute. or DLA is an option to apply for if you don't get it already.
So hope that helps to get you started.
Oh and do ask the school for all their info - you may find them less helpful if they find out you are going for an EHCP, so keep them on side as long as you can, and keep quiet about the EHCP for now, just in case it clouds their judgement.