I can't comment on the legal situation.
But moving forward, the current good practice in schools is that is you are going to restrain a child then someone who is qualified in positive handling (descalation and restraint) is needed.
The course can be done online.
alison.com/course/positive-handling-and-behavioural-training-in-schools
However doing this kind of thing online is very different to doing it in real life.
The standard procedure these days in schools for a child having a meltdown (ie any violent situation) is
1 try to descalate. This might be verbally and it would involve not getting in the child's personal space at all.
2 if de-escalation has not worked, remove the class from the classroom and leave the child in there with one adult. (Minimises harm). I have done this on multiple occasions.
3 never get between the child and the exit. If the child wants to run let them. In schools these days they are usually secure - so the child cannot exit the premises and there is limited harm they can do to themselves. Again, usually an adult will (try!) to follow the child from a distance.
4 the restraints specifically taught in positive handling can be used if all else fails. I have been in education twenty years mostly working with teens with send and I've never seen it done. I've seen 1,2 and 3 on a daily basis. (Well, not all but you get the idea).
The reason schools are so reluctant to use restraint is that it tends to escalate the situation and an autistic teen in full on meltdown mode can hurt a lot of people very quickly, and also because of the risk of legal action.
My personal advice would be that you need to choose one of two paths.
Either react as passively as you can to a meltdown - walk away to a safe distance, be clearly and obviously verbally calming him down, no physical force at all
Or
Learn from an in person course how to defend yourself using blocks and other techniques. There are self defence classes and martial arts classes that will teach you this. It will also help you control the panic at being in a physical situation.
The in person course will also give the same advice though -walk/run away unless you can't, de-escalate if possible.
You may be reaching the point where you physically cannot stop him bolting if he has a meltdown.