Hi, and welcome.
I have a DS, aged 6.7, with ASD and ADHD who is doing his first week back in school full time since Christmas after a very difficult year.
There are several things you could do while you're waiting for help from CAMHS and the hospital (because even when services are good, it can be a frustratingly long wait.)
Your DS obviously has some sensory issues going on, so I'd recommend reading the Out of Sync Child.
I'd ask the school to keep a record of all his outbursts and meltdowns and the events leading up to them so they can identify triggers within the school environment, so that they find it easier to find ways of avoiding them. The information would also be incredibly useful if you apply for a statement for him, in the future.
Look out for The Incredible 5-point Scale (both books are on Amazon). Your daughter may find it helpful, too, since it deals with stressful situations and how to identify and communicate and ultimate avoid the panic involved. It's a good one to share with school, too.
Given all this information, ask the school if they could provide a quiet space for him, maybe allow him to retreat to a less busy corner of the classroom, where he is not alone, but no longer surrounded by other bodies and chatter from every angle.
I would also cut out the computer games. The paranoid mum in me would be at least a tiny bit worried that the pallor and unresponsiveness might possibly be something untoward going on, anyhow, so even if he wasn't getting so uptight and aggressive when he's playing them, I'd be worrying about that, in itself.