A victims advocate can do a lot for someone in his position. Call Victim Assistance @ (765) 342-1186 and ask for a referral to a victims advocate for a witness to a shooting. I think that's the right number for your area anyways.

If you would rather talk to him then just use good ol' psychodynamic theory.

Simply put:
1. Create a safe situation.
2. Let him talk about it.
3. Invite and allow him to feel and express their feelings.
4. Help to come to a resolution in due time.
5. Use this as a leaning situation to develop skills in dealing with feelings in general.