Agree with other posters. Request an EHCP needs assessment immediately OP. You can apply on behalf of your DS yourself. it will in effect mean the local authority have to provide and fund education that meets his needs until age 25 or until he achieves the outcomes in it. This can include specialist autism colleges that he can do courses up to age 25 that would provide education and work experience.
I had to do this after my DS’s secondary said it wasn’t necessary. Our local authority twice refused to assess as they said his school hadn’t shown what support he was getting (none) but on the 3rd attempt, after I made loads of complaints, he got at EHCP at age 15 without having to go to Tribunal. I now bitterly regret that I didn’t go to Appeal after the 1st refusal when DS was 12 as mainstream secondary school was a disaster for DS with long term affects on his MH, but we parents of SEN children are often made to feel that we’re in the wrong and the so called professionals know better!
IPSEA are a nightmare to get through to for advice but you can try and book an appointment on their website or try the call in helpline (go on the website to check times). They have templates to request assessments, advice about appeals etc. I think Appeals to the SEND Tribunal are running at about a years waiting time so start the process quickly in case it comes to that.
Think about what DS’s SEN needs are. Does he have any learning disabilities/difficulties, processing delays, MH needs (anxiety?), social skills, where is he at with independent living skills - budgeting money, personal hygiene, can he cook, do his own washing, operate machinery, keep himself safe in and out of the house?
All that can build a picture. Also apply for PIP. I didn’t think my DS would get it but he did with only a short home visit.