Trained ASD specialist 1:1's can be provided in MS, but whether DS' 1:1 will be depends on his EHCP. What does DS' EHCP state about the number of hours, experience and qualifications of the 1:1? Is section F detailed, specific and quantified, or is it vague with phrases such as "access to" "or equivalent" "would benefit from".
If the training and qualifications are specified in the EHCP the 1:1 must have them. If they don't you can complain - both to the school and LA, as it is the LA who are ultimately responsible for ensuring provision EHCPs is provided. If necessary, you can force an appropriately qualified 1:1 to be provided via Judicial Review. However, if the EHCP is vague it can't be enforced, and you should focus on making it specific and quantified.
What the school are doing is illegally excluding DS. Remind the school part time timetables are illegal whether parents agree to them or not. They are only legal if short term working towards reintegration, that clearly doesn't apply if after 18months DS is still only attending 1 hour a day. Inform them he will be attending full time and you won't be picking him up early unless you have official exclusion paperwork. Again you can threaten Judicial Review if full time education isn't being provided, and depending on specifics may have a Disability Discrimination case.
Does DS' EHCP contain other provision such as SALT? If so, is he receiving it?
Lastly, do you think DS' EHCP meets his needs or do you think you need to ask for an early review or reassessment of needs?