M.L.
I imagine that the deep breathing won't work as well for a child as it does for some adults. He may need a physical activity to release the stress of strong feelings. Now, I'm assuming you've taught him well about not taking his anger out on living things. But if there's such a thing as a punching bag for a six-year-old, that might be helpful for him. Or he could take his aggressive feelings out on some modeling clay that he could pound. He could run around the back yard six times as fast as he can. (He could get his siblings or his friends together and race, maybe.) If he's artistic, give him lots of big paper and paints or crayons and a place where he doesn't have to worry about not making a mess, and let him express his anger in his art. Maybe once he has a safe emotional outlet (or more than one), then when he has once settled down he will be able to talk more about what bothers him. Communication has to be a part of relieving anger - when he's at the point of being able to communicate. Hope this helps.