My wife and I are trying to provide as much support as we can. What should we tell him? He doesn't want counseling. Should we force him to go? Should we remove him from school, or make him return? This is all very confusing.
All of the behaviors he is experiencing are normal: Sadness, anxiety, and depression. He needs help figuring out how to get out of this black hole. Removing him from school on any kind of extended basis would not be a healthy response. He needs professional counseling and he needs it preferably from a therapist who sees many kids his age and who has dealt with teen suicide before.
If your son was not the only one who witnessed this attempted suicide, there may be great benefit in treating other kids who witnessed it in a group counseling setting, as well as having your son receive individual therapy. Although he says that he doesn't want counseling, as his parents, you must insist that he receive it. He is exhibiting too many post-traumatic responses for you to just leave him alone until he "snaps out of it."
Research as many reliable medical and psychological resources available to you to find therapists who are a good match for your son. Let him interview a few of them and then select the person he feels most comfortable with. Expect continued resistance on his part about seeing a therapist, but tell him that you cannot stand by and see him suffer like this and that it is not a sign of weakness or "craziness" to have these reactions. Offer to attend the first session with him, at least for the first part of the session.
Contact some of the other parents whose kids also may have witnessed the suicide attempt and see if they are experiencing similar behavior with their kids. This is a situation that is not going to get better on its own. Get your kids the help they need now.