This is a job for a professional. Speak to him as a friend, let him know that you will be by his side as he goes to speak to someone, provide suicide hotline numbers.
From
http://www.stopasuicide.org/actdecision.aspx
1.
Do take it seriously. 70% of all people who commit suicide give some warning of their intentions to a friend or family member.
Do be willing to listen. Even if professional help is needed, your loved one will be more willing to seek help if you have listened to him or her.
2. Care
Do voice your concern. Take the initiative to ask what is troubling your loved one, and attempt to overcome any reluctance on their part to talk about it.
Let the person know you care and understand. Reassure them that they are not alone. Explain that even if it seems hard to believe right now, suicidal feelings – although powerful – are only temporary, and that the usual cause (depression) can be treated.
Ask if the person has a specific plan. Ask if a suicide plan exists, and if so, how far has he or she gone in carrying it out? (Please note: asking about suicide does not cause a person to think about – or commit – suicide. This is a myth!)
3. Treatment
Do get professional help immediately.
If the person seems unwilling to accept treatment...
Call 1-800-273-TALK (1-800-273-8255) or a local emergency room for resources and advice.
If the person seems willing to accept treatment, do one of the following…
Bring him or her to a local emergency room or community mental health center. Your friend will be more likely to seek help if you accompany him or her.
Contact his or her primary care physician or mental health provider.
And if all else fails... call 9-1-1.