5 yr old DS been sent to headteacher today

[Fishwifey]

His behaviour is getting worse. In school today he managed to injure 2 kids who had to go to 1st aid to treat bumps to the head. He’s been hitting, shoving, fighting with kids in school and I’m so upset. My boy is so sweet and loves snuggles with me. But he shows no remorse for this awful behaviour. I’ve sent him up to his room(no toys in there) as I just don’t know how else to address this. We’ve already confiscated the iPad, screen time has been significantly reduced lately too. He just doesn’t seem arsed though!!
Please, someone tell me what to do. I have 2 other DS and we had planned on a lovely festive weekend. I don’t want him to do anything ‘nice’ this weekend but I don’t want the other 2 to miss out because he’s been a shit!

What does he say when you ask him about it ?
Does he know what he has done / why he is being punished ?
If so, can he tell you why he did it ?
Is this Reception or Yr1?
What was his behaviour like last year? (In Reception or Nursery)
What is his behaviour like at home ?
Is there anything else going on that might have unsettled him ?

School ought to have some idea of why this happened I'd have thought (ie what was the context/immediate trigger each time) - what have they said?

No real trigger from what they say. It he said ‘they were annoying him’. These outbursts happen in the locker room so teacher is talking about banning him from going in there.
He’s yr 1. He was absolutely fine last year. This started about a week after the new term. He was pretty miffed that there were no toys to play with and said it was ‘rubbish and boring’.
Teacher said the other day that after hitting a kid he didn’t really remember it. I’ve asked why he’s done it and he sort of shrugs. Does he just ‘see red’ perhaps?
Behaviour at home is better, but by no means perfect. If little brother starts up, he'll retaliate and, unless stopped, will get angry and really want to get stuck in and get his own back!

I'd keep in mind this is a long term, and, from now on, lots of exciting things going on and out of routine, and a LOT of little ones struggle, however, as you say it has been going on all term, then I'd ask for an appointment with the teacher to see if there is a way you can work together to support him.
Work on finding out if there is a pattern - certain times of day /certain activities / when asked to do something in particular (maybe something he is struggling with?) / when it is with other children / when he has to work quietly / when they've been sitting for a long time without chance to move around / on 'wet play' days ???? Then it is easier to put strategies in place. Also see if they can set up some kind of monitoring / reward type system for managing the times he finds difficult.... small steps, instant feedback. Then let him know that you and MrsX want him to be happy and are trying to help him and are going to ..... {insert strategy} when they notice him trying, and, when he feels angry let him know he can .... {insert strategy}.