Some special schools do GCSEs.
In your position I would look at all the possible alternative schools, both near and far. Including some mainstreams schools which are set up to cope, special schools elsewhere and the independent sector.
Are you in a position to move? He may be fine at this stage of school, but once reading and writing are expected, and everyone around him is able to do this, he may start to feel isolated.
The idea that 'inclusion' is best served by all children attending mainstream is so misleading. If the school can't, won't, doesn't meet the child's needs life is very tough.
I have a child with autism in mainstream secondary, which is the right setting for him (at the moment). Since he was diagnosed I've discussed all our local options with the parents I've met at local autism groups and training, and every parent has had to make a decision based on their own child. Several of them recommended the school my son goes to but even similar 'little professor Aspies' struggle in the wrong mainstream setting.
It can be very difficult to get an LA funded place at the right independent school but if that's the right setting for your child, it is worth the years of grief it would take you to get a place.
A woman I know locally has a boy with autism and a small amount of learning disability. Not disabled enough for our excellent local special school (which specialises in children with very more complex needs) and unable to cope with mainstream even in the year below. She managed to get a funded place at a special independent, including his taxi there and back every day. It can be done.