Hi, I'd like to help you with your question.
It sounds like your son has two problems going on here. One is the depression and the other is the alcohol use. It is difficult to say how severe his depression is since he is using alcohol. As you may know, alcohol is a depressant and can cause depression to deepen and get worse. It may be what drove him to become suicidal.
Your son needs an evaluation immediately. Take him to the ER and let them know he is suicidal with a plan. If your son will not sign himself in to the hospital, then you may need to 302 him, which is an involuntary commitment. He needs help right now so it is important he get it right away.
He then needs to follow through with an alcohol treatment program. Hopefully, he will be willing to see that he has a problem and will work on it. The biggest obstacle to his treatment is if he is in denial about his alcohol addiction.
The doctor will most likely put him on medications and it is important that he follow through with any recommended treatment. It may take a while for him to feel better, but if he follows through, he can recover.
Contact Al Non- http://www.al-anon.alateen.org/ - for assistance for your family and on how to help your son. There are groups you can go to for support and ideas of how to help.
I hope this has helped you,