We've been through similar situation in some respect, but already with a diagnosis and a statement/ EHCP.
I am sorry I don't have any good answers, even for my DC. I would just share what I can say. It is not necessarily the case that the school and CAMHS won't help, but that you will have to work very hard on them helping your DS. ASD is always an uphill long term struggle.
In my experience once anxiety and meltdowns start, you need to deal with that first, remove the triggers, otherwise it might escalate to school refusal and crash out. Tell the school that you are trying to prevent that situation, that the stress and demands of GCSEs have triggered new needs, a developing crisis.. I am not sure it is possible to have 'a happy and secure child with no grades ', the grades situation will feed the insecurity, anxiety and low self esteem for years ahead. Ideally, you need to have a plan that salvages both. Do you know what triggers the problem? He might be used to doing little at home, while new GCSEs require a lot of practice and homework and writing in lessons, so he is falling behind. Can you make a revision and homework schedule and revise with your DS? It might just give him the confidence and control he needs. It might be the only quick fix. Can you afford tutoring?
Try reading the 'starting secondary school' thread on this board, approaching 1000 posts, Penguinpanda's posts might give you some ideas of what the school could do that works.
You need to put ASD on the table. Try to calm down and make a plan. Do you have local parents support group / networks for SEN? Or at least Parent Partnership? Try to find out what can be done in your area for ASD/anxiety in year 11, what the dx threshold is, who are the gate keepers (names) for diagnosis and for SEN support, for anxiety support, what has been done successfully in your situation. You don't have to take what they say, but it will give an indication of what the system for ASD is in your LA. Talk to SOSSEN, they always have very pertinent advice. You need to figure out what you need the school to do, what you need the NHS to do and what you have to do to make it happen.
First Senco, they don't have to have a diagnosis to start helping. In addition your DS already has a diagnosed condition, so they ought to be talking to you. Dropping grades should be visible at school, bed wetting is a very serious symptom, they can't just ignore you saying it. Masking and meltdowns at home are typical for ASD. Go back to Senco. Expand the conversation to ASD and anxiety. As Maisy said, find teachers who would see things, but just can't put a name to what they are seeing, help them to understand it could be ASD and it is anxiety. Meet with them or ask a joined meeting with the Senco. Document all problems, show there is a pattern for ASD, show what the triggers are, show what helps, put it in writing in a letter to Senco. Does the Senco understand ASD? Talk to her passionately, touch some emotion. Sencos usually have children with SEN. Do they have a school counselling service? If necessary get a meeting with the head of year and the headteacher. You want to get through to them that you have a DC possibly on the spectrum that has an escalating anxiety and is not coping, and you need an emergency action plan, to prevent your DS crashing out of GCSEs. You may need to define what you want them to do. Do you need an urgent sensory diet (OT), provisions made in form of small group and 1:1 support, safe space at school to calm down and do some work, revision and homework support, anti bullying action? Do you want them to write something about failing grades, call EP and OT assessments? Take notes, document everything. You will need to go step by step through the SEN processes.
Make a diary of what happens every day, does it show a pattern for anxiety and ASD? Go back to the GP ask a referral to the paediatrician, to the psychiatrist re ASD. Continue keeping your diary.
Obviously an independent diagnosis and assessments for Psychiatry, OT, EP, ADOS are available and could be the way to go...
Is there youth counselling service in your area? Would you be able to pay?
Unless you are prepared to home educate, you need yourself at home and then the school to diffuse the triggers and put your DS back on track and this means you need to go through the SEN processes.