It sounds like your son is struggling in this particular classroom, and the situation is getting worse instead of better. Given that he’s never had behavioural issues before, something isn’t right. The teacher insists there are no triggers or patterns, but that doesnt mean theyre not there it just means they havent looked closely enough.
You need a proper meeting with the teacher, headteacher, and SENCO, not just informal chats. Ask for specific details: when do these behaviours happen, whatever is going on around him, who else is involved? If they say there is no pattern, push back. Kids dont suddenly change for no reason.
Talk to your son in a way that makes him feel safe to open up. Instead of focusing on what he did wrong, ask what he likes and dislikes about school. Does he find certain times of the day hard? Does he feel the teacher treats him differently? If he is laughing when asked to do something, is it nerves, frustration, or is he copying someone else?
It is also worth considering whether this is a personality clash with the teacher or if the class environment itself isn’t working for him. He’s bright, so maybe he’s bored or feeling misunderstood. Given your older childs autism, it wouldnt hurt to explore whether your son has any subtle sensory or social difficulties that are only now becoming obvious.
Spending entire afternoons in the headteacher’s office isnt a solution, and banning him from football only isolates him further. The school needs to focus on helping him, not just punishing him. If they wont acknowledge that something’s not right, you may need to push harder - ask for a behaviour specialist to observe, talk to other parents to see if anyone else is having similar issues, or, as a last resort, consider whether this school is still the right fit for him.