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the school is telling you bare faced lies. It is possible if your child has special requiremants to sit the exam at a regional center where the needs can be met.

There would also be scope for information to assist in the exam to be listened to instead of music. I am not saying that your son would do this, but the school have to recognise the opportunity that exists if any child is listening to a recording. Audible music would bring issues of taste.

I am all for help for individuals with problems, but ultimately, we all live in the real world. One day, your son will go on to earn his own living, and at the moment, that real world does very little to accommodate people with asd. 

He needs to know his own strengths and problems, and how to work with them.

I worked in a variety of indoor premises. One was a bright, noisy, open plan office environment. I used to find myself "coming round" at intervals, as though I had fallen asleep. Having studied asd a little more, now I realise I was switching off from this overbearing environment, which was making me unable to function. I was moved, after six months, and the problem stopped.

I think it must be very difficult to find the ideal mix of support and "real world" to give a child the best chance of acheiving his potential.

I hope you do not find this too negative. There is joy to be had in this world when we find the right niche, but it helps if you know where to look.

It is quite possible for reasonable requests to be reasonably refused sometimes. e.g. I reasonably ask my boss for a pay rise but he reasonably turns me down. It is a reasonable thing, to ask for music during an exam, but it may also be reasonable for them to apply a set of rules across the board because it would be difficult to supervise an exam if half the pupils have headphones in and can't hear instructions from the invigilators.