I’m really saddened that they’re punishing him for this, given his recent diagnosis. I hope you’re coping okay OP, I know how difficult the diagnostic process is, having both my sons diagnosed in the space of 2 months earlier this year.
Are you getting much support? Is there a SENDIASS near you that you can speak to?
Have you met with the school’s SENCO? They need to be making adjustments for his additional needs and it doesn’t sound like they are to me - taking break time away from a child with ASD for something that was a one off and happened the day before is so counter productive.
Does he have an OT supporting him yet? Are there already things in place to support him at school while you go through the EHCP process (eg aids for sensory seeking behaviour and so on)? My boys are much younger but one is constantly sensory seeking and sadly one of those things is pulling his penis out of his nappy constantly unless his clothes make that impossible. Could he have felt overwhelmed by the lesson, is he sensory avoidant? Were they doing something in the lesson that was difficult for him and made him feel overwhelmed (eg proprioceptive movements - swinging, spinning etc)? These are all things, alongside the recent freedom from nappies (huge well done to you) that off the top of my head may be a factor.
You mention he has some PECS cards - how long has he been using those? Is he non-verbal or limited speech, and how’s his understanding? If his understanding and his expression are both limited then it’s even more important to use PECS as much as possible (my two are completely non verbal but only just starting as their understanding is so limited). Do check out the app I mentioned (Visuals2Go) and consider getting him a tablet that can run it, as then he can do it all virtually, and it even has a voice that can speak the words on the cards selected - are you getting SALT help to learn PECS?
Sorry for the essay, I hope some of this is helpful. Sounds like you’re doing an amazing job as a single parent with additional challenges.