I am not an expert and hopefully somebody more articulate and knowledgeable will be here to clarify the ins and outs of the law and the code of practice, just wanted to say that nothing should stop you in pursuing an EHCP if you feel that is wat your DS needs.
Given that he went a few sublevels down, (I assume it is documented and you can show it), it shows that "the gap is widening", meaning the school needs to put in place more intervention. It also looks like the school does not fully understand his needs as they let his levels fall and did not put intervention in all subjects earlier. They seem to be scrambling to do that now, but not in all subjects, probaly because they don't have resources to do it in all subjects. This is showing that the needs of your DS, in order to close the gap cannot be met within the school resources, hence he might need a EHC assessment to understand what his needs are, what intervention is required and an EHCP to provide appropriate resources.
The good thing is you can request an EHC assessment arguing that the "gap is widening" instead of "narrowing" (code of practice lingo) and his needs are not fully understood. It is already established that he has SEN needs (ASD).
If you google and download the SEN code of practice and look carefully in the part on assessment and how to demonstrate inadequate progress. There are several tests on inadequate progress, "gap not narrowing" is just one of them. You could figure out what case you could make with the evidence you have.
You might also wright to the school, rather than calling and ask for a, Provision Map or IEP review meeting to discuss his falling levels and what interventions they are putting in place, and why not in all subjects, ask whether they have used all resources "delegated to the school". Ask for a copy of their Provision Map to see what thours their provision adds up to, basically trying to get more evidence that the school resources are insufficient. Usually the school would support your application for assessment and EHCP. They may tell you that they need to wait and see whether the new intervention worked. That is corect in law, but given that they don't have resources to help in all subjects, it is likely that your DS needs cannot be met within "resources delegated to the school", hence need an EHCP.
So, given falling levels, basically it might be that right now is the best time to get an EHCP. And there is no downside to the assessment.