The first thought I had since this boy was your son's best friend is `does your son feel somehow responsible?' If the two were close enough that they often confided in each other, your son may be feeling that he somehow let his friend down by either not seeing the signs that led to his taking his life, or maybe he did know of his friend's personal demons that led to suicide, and your son may be experiencing feelings of guilt because he wasn't there to stop him.
I agree with the other poster, talking frankly but in a loving manner with your son about his thoughts and feelings is very important to do, and soon. Your role should be as supportive as possible. Allow him to open up as much as he can, and regardless of what he says, let him freely vent. I think it is really important for both you and him because of the complications that situations like this often present.
With surviving a suicide of a close friend, your son may be more psychologically vulnerable, than if this were an accidental/natural loss of a friend. Especially if your son does know and understand the circumstances that may have led to his friend taking his life.
I think it is important to find out in a non-confrontational way if your son feels he was somehow invovled and if he also may be contemplating doing the same as his friend. This is a very sobering thought. And, I'm not an expert in these matters, but I have read that often with suicides among the young, sometimes the young will do the same to cope with their own problems, rather than seek help for themselves if they lose a friend to suicide.
It sounds like your family is very close. If all of your sons knew this boy and his family, it might be nice to get as many of your son's older brothers back together to be with him and share the good times and memories of this friend.
Most importantly, they along with you and your husband could be a good support system for him, and help guide him as he begins the healing process after losing his friend. Don't hesitate to invite friends or others over too. Talking will be the best medicine, and the only way for you to gauge whether your son should seek more serious counseling.
If your faith is important to you, I would for sure make an appointment with your pastor/rabbi for "grief counseling". Particularly if the deceased comes from a like-wise religously strong household. Often, rationalizing suicides can be difficult when religion/spirituality was/is an important part of both the deceased and survivor's lives. A good pastor/rabbi will be able to help your son sort out feelings and concerns, and give both you and your son good resources that you may be able to share with the deceased's family as well.
Last, but not least, the other poster's recommendations of providing support to the deceased's family is a wonderful idea, and will possibly provide a good carthesis for your son. However, I'd recommend that both you and your son refrain from sharing any serious concerns or private knowledge leading to the suicide until long after the deceased's mother/family have gotten counseling themselves and seem to be on better footing. And I'd only do it in the presence and guidance of a professional counselor or pastor. They could (counselor/pastor) also help you find the right approach to sharing this information in the most sensitive way possible.
I'd suspect, many of the same questions your I suggested your son may have, may be the same for the parents of the boy, but intensified. Especially if there was any strife between them about school, family issues, or other unknown issues. Just be careful, because the parents will be supersensitive for a very long time about their son's death. They may even become reclusive and non-communicative due to guilt or other related feelings. Just read the signs and make it known that you will make yourself available as need be.
Your son and his friend's family will be in my thoughts and prayers.