I am a secondary SENDCO and specialist assessor for access arrangements AND have a son with ASD as well so know this scenario well.
I genuinely believe that the school will not be trying to be difficult about providing arrangements for your son but it is becoming increasingly difficult to physically organise where students with additional concessions can be placed within the school, with the limited resources they have. We have had more students with access arrangements each year since i started there in 2015.
The additional challenge is the shortage of invigilators and then the parameters that some of them are prepared to work within and as its hard to recruit new ones, schools are being forced to accept their preferences.
In my school TA’s have to support our EHCP and vulnerable students as the invigilators aren’t comfortable with anything that isn’t a classroom or larger space for example.
I would recommend going in with your understanding of how increasingly difficult our SENDCO jobs have become and to lay out the evidence for his needs and why an individual space might be the best solution for him. However I am afraid reasonable adjustments are just that and finding a seperate room for one person is not necessarily ‘reasonable’ if they are working in classrooms every day with a range of other students.
Maybe trial your son sitting mocks in the smaller room with a few others to show that you are trying to work with their limitations and then see how this goes.
It really tough as all schools want to ensure the best outcomes for All students but as we know there is a finite amount of money and resource to do this with and sadly that may mean a compromise.
he can wear earphones to help mute noise if this is helpful to him? I have had a student who had white noise on as well. Maybe invesitage with him what might help support his need for quiet etc before the meeting.