There's a couple things you can try. One think you can do is keep a journal of all of his "incidences", dates, times and what happened. You can also have the 12 year old ask him why he's angry all the time, sometimes when it comes from a child it makes more of an impression.
As a last resort, you can start videotaping him. You can have it where you tell him flat out that you're going to start video taping his blowups, or you can videotape him without him really knowing and have sit down in a couple of weeks with highlights.
Many, many men and women think that to admit to a mental problem, regardless of how slight is to admit that you are damaged or in some way inferior to others. It can be embarrassing, intimidating and most of all scary. The most important thing is to present a united front with the entire family. Usually if more than one person stands together they may take it a little more seriously. Let him know that you all love him so much and would not come to him about it if you did not care.
I welcome your thoughts, let me know if you want to talk more