How to help someone with anger issues is a tricky question. To calm down a person who is already suffering from anger issues cannot be an easy task. It’s better to take him to places where he can relieve all of that hostility and tension.
Or, alternatively, take him out to a nice, peaceful dinner every now and then. If you are truly his friend and care about him, say this to him.
Also, avoid going to places that trigger negative reactions. Avoid areas that are full of crowds and irritating noises that cause stressful situations. You want to keep your friend calm at all times.