Actually a young person can qualify for legal aid at age 16 - worth getting him to a solicitor. Mine signed his legal aid application on his 16th birthday!
Post 16 is a very grey area. LA try to pass the buck to Connexions but Connexions are funded (or were when I was in this situation) by the L&SC who expect the LA to continue to provide post-16 education.
So an impasse is reached.
There are some things you can do:
Write a letter for DS to sign stating that he wishes to remain in full-time education at school post-16 and send it to the LA. That puts his wishes on the record.
Look around for independant placements that may be suitable with a view to getting it funded by either the LA or LS&C. That will prove that suitable provision is available - at a price.
To get an indepenant placement you'll need to prove that not only the LA but also local FE College could not support his needs so look at all the local FE courses and ask what support their Learining Support Team could provide.
MP will be a waste of time. You may get some help complaining to your Local Councillor.
It's actually quite easy to get LS&C fubnding if you have a well-documented need. You can also ask any suitable independnt schools / colleges to assess your DS's suitability and their admissions staff can assist you in securing funbding for a placement.
You can initiate a formal comapkint against the LA and eventually to the LGO. Worth starting the LA complaint now as these can take months.
Other than that complain, complain to the LA Director of Education, Cheief Executive of LA, Local Councillor, Regional Director of Connexions.....
Unfortunately many on thsi board will face this same dilema at post-16. It's horrific, unjust and inhumane. But to them our kids are just expensive hot potatos that they try to drop as quaickly as they can.