How to help my brother get through seeing someone commit suicide?

It's very early days, though I agree this is something you'll want to keep an eye on. I have to say, since we're talking about extreme situations, that after 9/11 I was jerking awake from deep sleep for a good couple of weeks, though; so I'm not surprised that your brother is finding it hard to see past the shocking images he's just experienced so up close and personal.

Has he made a full statement to the police?

I'm sure the police got involved and I would bet that they have some numbers of some counselors your brother can call.

This happened to me. I was on rte 13 in Maryland just as the highway widened to 4 lanes to join with Rte 50, the speed limit 65mph. At this point 3 teenage girls decided to cross the highway. I saw this in my rear view mirror, the first girl hit a vehicle, and went spinning in the air. You just knew she was dead but the way her head flopped, This kept replaying in my head, no matter what I was doing. Weeks went by and it was still as potent as the first day. I know this is morbid and black humor, but it worked. My neice started humming "There she goes, there she goes again", and it worked it removed the image and I started to heal.

This is called post traumatic stress disorder. You are going along, normal day, and something happens that is unexpected, and horrifying. He needs a therapist who can help him erase the image. Seven years later the image of that poor girl, still haunts me at the most unexpected times, but I am okay.

You are wise to be concerned, keep him busy with tasks that need concentration, so he gets some relief from the memory. Encourage him to see a therapist. He can suffer for months trying to heal by himself, or a few visits to a shrink will give him comfort and perspective, and give him the skills to put it behind him.

See if your local government has some type of victim/witness assistance program for counseling or other assistance. He may develop PTSD, but please realize that many of us experience trauma, incorporate it into our vast number of experiences and move on. Give it some time and don't bug him about "talking about it".