Assuming for the moment that you are willing to put in some time...

Would you consider taking a video of this behaviour? Short, maybe 30 seconds- 1 minute. Keep it for a time when he is "happy" and when you can, take a moment to watch it privately with just him, (even just on the screen of the camera/phone.) I tried this once with a child who was having an issue (I used a photograph) and it helped.

Seeing what the rest of us are seeing in these moments can put it in a new perspective for the child. Keep the tone serious but not scolding. I don't think I would recommend taking a video every "bad" day. I think I would use the same one over and over, if necessary, to avoid making it an attention-getter. Use it as a starting point for the conversation. "This is what I saw, yesterday. How come you were so angry?"

That's my first suggestion. The second thing, give him alternatives. What could he have said in that situation? How should he have handled it? IF he can tell you why he was angry, you should be able to give him more appropriate actions or words.

The last thing, practising and modelling. Show him what you're talking about. Say the things you want him to say and have him repeat. Maybe make a short video of that too, to compare.

Maybe that all sounds like a lot of work, but I think I agree with 5LM, if no one takes the time with these kids, what will they be like as adults? Yikes!

One last thought, (I won't even get into my rant about video games...) does dcb have older siblings? Older kids he hangs around? That could be a clue as to where it's coming from.