How can I calm my boyfriend down when he's mad?

He has anger issues. He gets anger fairly quickly and it takes him a while to calm down. He never takes it out on me, which is a big relief. It seems that when he is mad, I just sit there scared of his next move. How can I help him? I find scratching his back helps but what can I do when we are out in public or in the car and I can't do that?
How can I calm my boyfriend down when he's mad?
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