IMO, you would be daft to ask the LA to cease to maintain. It also doesn’t sound like any of the legal tests for ceasing are met.
However, that doesn’t mean doing nothing.
I would request an early review. On their website, ISPEA has a model letter you can use.
For many DC whose primary need is ASD, a SEMH setting is completely the wrong place.
Have you looked at other placements? What therapeutic provision is in the EHCP?
If it is inappropriate for provision to be made in a school, there is also EOTAS/EOTIS.
If DS is unable to attend school, the LA is still responsible for ensuring he receives a suitable full-time education and the provision in the EHCP.
Personally, I wouldn’t EHE, but if you want to, you can still do that with an EHCP. Because DS is in SS funded by the LA, you would need to request permission to deregister. That isn’t quite the same as permission to EHE. And permission must not be withheld unreasonably. If you EHE, the LA doesn’t have to provide the provision in F of the EHCP.