How do you help someone from severe depression?

My wife and I have been trying to help our kid cope up from severe depression. He is just 20 years old and he has been through a lot. We already talked to him about the reason why he's been in so much grief these past few weeks, but we receive no response. He's not going out from the house since. He lost so much weight. He's not eating. I talked to some of my friends about the possible causes of depression, but none of those helped me, except for this top foreign dating coach which states some of the glaring reasons of this mental issue. But I know some of you here knows this, I need someone who has the same experience as my son has. Please help!