You may already know all of this, but when he’s hitting out he’s essentially in fight/flight mode, so trying to explain anything (or expecting rational behaviour) will not work – he’s reacting, not thinking. Pick a time well past the incident when you are both calm, perhaps doing something side by side like riding in the car or playing with lego, and casually use some open-ended questions to invite him to reflect on what happened. (Yes/no questions do not invite introspection or creativity!) Be very alert to anything that might sound blaming or shaming – kids are so sensitive to that. For example: “What was the best part of your day today?” “What was the worst?” “What were you feeling when you scratched your friend?” “How do you feel now about what happened?” “If you could go back in time, what would you do differently?”, etc. Be empathetic with him – he no doubt feels bad about losing control. You can have compassion for the part of him that gets so mad it wants to attack others, while helping him think of things he can do instead when he gets frustrated or angry. Having a rehearsed response and planning ahead is so helpful. Teach him about “flipping his lid” (Dr Dan Siegel) so he has language for it. Good luck!