I wrote a letter to the LA when my school was asked to take a child with an EHCP, but only because we couldn’t meet the needs of the child as required by the EHCP.
There was no lift in the school and the child used a wheelchair, so half the curriculum subjects would have been inaccessible, because there was no way of getting from the ground to first floor, other than stairs. The EHCP also detailed a particular therapy on a daily basis and we didn’t have a therapist in school and to ‘buy’ one in would have cost thousands per year to meet the terms of the EHCP. There were a number of other reasons too, all of which contributed to making the placement not appropriate.
As far as your question is concerned OP, it doesn’t sound as though your son needs such a concentrated level of support, so they need to explain fully why they say they aren’t able to meet his needs. The money could be used to support any adjustments that need to be made because of the ASD diagnosis, but that is what the EHCP is for and some things are perfectly possible to arrange with willingness. Ear defenders for a child who doesn’t like the school bell, leaving a lesson a few minutes early, a visual timetable, preparation for ‘oops’ moments for example.
I think you have to badger the case worker to badger the school for a response. And contact IPSEA as a PP has said.