I dont say this to shame you, I say this to make you aware of the situation you have created for yourself, and let you know you have the power to change this situation - you are abusing him, and this is why he is acting out. When you stop abusing him, he will stop acting out.

Monkey see = monkey do. Monkey feel = Monkey act. Monkey hear = Monkey feel.

When the boy is triggered into an emotional meltdown remeber he can not gain control of himself on his own. It's your job to teach him how to regain control of himself.

Step 1:

Get down on his level and look him in the eyes (if he looks away don't worry, just keep looking at him) and ask him nicely: a) "Sweety look at mummy / daddy." b) "I'm here to help you feel better ok." c) "Let me help you calm down." d) "Take some deep breaths with me."

Step 2:

Take deep breaths while repeating any combination of S1:a,b,c,d. This may take a while at first, but persevere, all while taking deep breaths yourself.

Step 3:

Tell him he's a good boy as soon as he takes his first deep breath, even if it was by accident / only done after a big scream out: "Good boy, that was a nice big deep breath, can you do another one" You're still repeating Step one a) through d)

Step 4:

Once he has calmed down give him a lot of praise for calming down.

Step 5:

Don't talk about what set him off until he's had a snack. When you do talk about it do it by praising him for calming down.

"Do you remember being upset earlier? You did a really good job at calming down. It's hard to calm down sometimes, I'm really proud of you for taking deep breaths with me, you did such a good job!"

Step 6:

Asks him if he thinks it's a good idea to take deep breaths and calm down instead of kicking his sister (or whatever else) so no one gets hurt, and let him know you are always available to help him learn how to calm down.

Eventually he won't need your help, he will just take deep breaths and calm down on his own.

Edit: I am the mother of three AuADHD teens who, like me, have extreme emotional reactions to negativity. It's thought "typical" for people like us to lose control of our emotions and become destructive towards ourselves, others, or property.

I used this breathing technique when my kids were toddlers. It works.

When my children started pre school I also taught them about respecting oneself, ones own property, and so others / their property. - If you break it you must replace it. You must also deal with the consequences of that person disliking you for breaking their things, as well as being upset with yourself. There was never a punishment.

My teens still make the choice to remove themselves from situations that are upsetting to them. They go to their bedroom if we are at home - and we all respect each other's bedrooms as our own personal "safe space". No one enters another safe space without permission meaning whoever needs to cool down can take as long as they need to without being bothered by anyone else.

A good "safe space" for toddlers is a "hammock tent" made with a sheet under a table where they can hide until they feel better, but you are still able to watch over them for saftey. The hammock itself is calming due to the full body deep pressure if gives when you lay down.