My 4 year old keeps hitting children at nursery and now a parent has put a complaint in

This has been an ongoing problem for over a year now. My son is neurodivergent but no diagnosis given as of yet due to 18 month wait list(and before a mod deletes this as me asking for medical advice, I absolutely am not, however this is vital to state), and over the last few months, his behaviour has been escalating.

At home, he’s an only child and we don’t have too many issues, we do still teach him to share and he does play with his cousins as well. But when he’s in nursery or even at soft play, he often starts lashing out and hitting. This past week week alone his behaviour has been almost ‘out of control’ at the nursery and they’re struggling to identify a trigger and my son doesn’t seem to identify any triggers either when I’ve tried to find out from him what’s happened, and the staff believe he’s doing this for attention more than anything.

Anyway yesterday, a little boy was waving bye to his mother and my son went up behind him and pulled him back by the back of his neck. And the day prior, he did a similar thing where a little boy wasn’t sitting properly for circle time and he pulled them down by putting his arm over their neck. As a result, the mother of this boy waving bye (rightly so) put a complaint in. We’ve explained this to our son quite sternly as well why this behaviour is unacceptable, we’ve even confiscated his toy and this still hasn’t made any difference as he just doesn’t seem to understand, resorting to laughing instead when I try and tell him off.

He gets 1:1 senco support at nursery and he’s apparently so good during that, and even when he was doing an activity with one other child yesterday, he was so good, but the moment he’s back in a group, he acts up and hits, and even hit his teacher yesterday which he’s never done before. His teacher also said he was sat on her lap yesterday and randomly closed his eyes and gripped the side of her legs for a moment before releasing, almost like an urge she said which she handed over.

I’m not sure what else I can do and any advice to approach this and why he could be doing this?