Help and advice needed- 
My best friend and former boyfriend of 6 years is dealing with intense depression. I honestly think he’s clinically depressed, we’ve lived together for 4 years, we have built a life together and he’s moving out, says he needs a change of environment. My 9 year old son (former bf is not the father) is extremely attached to him. My former boyfriend has known my son since he was three and of course as much as I’ve tried to explain this situation to my son he doesn’t understand what depression is and doesn’t understand why he is moving out. I love my former boyfriend and want to help him but my sons life and emotional state concerns me. He’s had a lot of changes at his biological fathers house we share 50/50 custody ( biological dad has gone thru 1 divorce and two different women in my sons 9 years of life). So my life with my former BF has been the only constant that he’s had. 
Here’s where I need help. My first reaction to this news is to cut contact with him once he moves out- it’s too confusing for me to process this whole situation (it feels personal at this point, even if my former BF is saying that it isn’t) and for my son as well and I don’t want to give my son (or myself) false hope. 
But he is asking me to be his friend and help him thru this and asking me to not cut him off (yet telling everyone that the reason why he’s moving out is because we are breaking up) 
He doesn’t want to see a therapist, or get counseling, or even go to a support group, he’s not telling his family that he is going thru this, I’m the only one that   knows how he’s feeling. 
What do I do in this situation. Do I help him thru this and keep this connection? Do I leave it to him to handle this situation by himself? Should I tell his family? 
I’m open to hearing any and all advise, from people that have gone thru this experience or from people who can give me insight into depression to help me understand. 

Thank you in advance for your help