4 year old biting for 1st time

[JammyGeorge]

Please help dear mumsnetters! Sorry if it's long.

Background - ds1 is 4 years old and started reception in sept. Up until November time there were repeated incidents of pushing, hitting and hurting other kids at school. At first it was all day, then it was lunchtime only and in the past 3 weeks there has been not one single incident of aggression. I was heartened by this and was hoping that it was a boundaries thing and he was coming in line, teachers really pleased with him. No problems at home at all. No problems in pre school. No problems in after school club.

Although he does hit pretty much every marker for ADHD on paper, but I'm not sure if his restlessness etc is within the realms of normal. I'm thinking if he did have ADHD surely he would hard to live with? I'm certain there's some sort of sensory thing going on.

Anyway, last weekend went to a party with an entertainer and he was fine. This week party with bouncy castle and music for 2 hours solid. Started off ok few tears over balloons by the last half hour his behaviour fell to bits.

A boy pushed him over totally unprovoked, he reacted grabbed him and was dragging him across the bouncy castle by his shirt. A couple of similar incidents. I took him out told him off made him calm down. Went back in and him and a boy were intertwined sliding down the slide, didn't seem to be fighting or angry, the boy burst out crying, ds1 got up smiling. He'd bitten the boy on the leg. I was mortified, apologised, made ds1 apologise. I then took him straight home. No party bags or cake or any of that. I've sent him straight to his room.

What do I do mumsnetters? How do I hammer home that you don't bite? I'm not normally shouty but I have shouted at him I was so annoyed and shown up.

He's up in his room, how long do I leave him there? What do I say to make it go in? I'm so scared he's going to turn into a biter on top of everything else.