Finally, if I were you, as pp has recommended, ask SS what they are going to do about transition, and request an (re)assessment for social care and support with a view to adulthood; but once that is underway, ask your CCG for the checklist for adult continuing health care (CHC) funding to be done too. DS might reach enough of the domains for cognition, communication, etc to meet the criteria.
Look online for the NHS guidelines on Continuing Health Care Funding for Adults, irrc it has the checklist in it in the appendices somewhere.
The big advantage of NHS CHC funding is that the NHS is free at the point of care, unlike social care, so DS won't have to pay any of his benefits towards it. If DS's social care is funded by social services (SS), the LA takes all the benefits and leaves the young person with about £26 a week to cover clothes, toiletries, holidays, a new television, going out, etc - I've been told its not enough. Even with NHS CHC funding, if DS were to live in a residential care home, he could still get his DLA mobility, although DLA care ceases except for the days, he is in your care or comes to your home for family visits.