Oh no. I am genuinely absolutely gutted for you but please DO NOT give up. You know your child better than anyone and in your heart know what they deserve.
Does he have an official diagnosis and if so does his diagnosis report mention masking etc? Research has proved that children who mask at school are more likely to have more meltdowns and more troublesome behaviour at home because they have bottled their frustrations and anxiety up all day.
I would definitely do a MR and send in as much information about masking as you can find. Literally dumb it down for them as they clearly need to understand that just because he can ‘manage it’ at school it doesn’t mean he isn’t struggling massively inside and then in turn at home.
I think I am going to get the same outcome as you. My daughter is diagnosed with ASD but like your son she masks a lot at school. Again like your son she is a high achiever so only a small amount of support is in place which is mainly based around social skills and sharing her feelings.
Please keep fighting - you will get what you deserve and it will be so worth it in the end x