Hello and welcome to the forum. It's a good place to read about other's struggles with depression and how they feel about getting help both professional and family.
I think one of the most important things for you to do is to get good information about depression. Beyond blue has a number of fact sheets you can download. Look under The Facts above and also under Get Support. It is good to be informed about a subject many people have little idea about and which scares many people because of all the misconceptions.
One very common trait in depression is to push away family and friends. There is a great deal of shame associated with any mental illness so the instinct is to hide what's happening. It may be your son feels he should be stronger, able to get over this on his own, or not wanting to worry you or be a burden. I doubt it is personal.
When you navigate through the fact sheets you will find a booklet written for family and friends. You can ask BB to send one or two copies to you. There is no charge. It has information that can help you with your son.
I also suggest you read through some of the threads in the depression section to get some idea of how people feel when they are depressed. No two people are alike but there can be similarities. I can understand your worry and concern about your son. For me it was the reverse. I was the person depressed and my (adult) children were concerned. I was still reluctant to talk about it even when I became suicidal.
What sort of concerns do you have about your son? It's good he is getting help, presumably from a psychologist or psychiatrist.
I have found what I most appreciated was someone listening to me. Good listeners are as scarce as hen's teeth. However, when it came to my family I found it very difficult to talk. After all I was their mom and I should be caring for them, not the other way round. So I also felt guilty. It may be your son has similar feelings. You have looked after him all his life and perhaps he doesn't feel he should put this on your shoulders. Do you think this may be possible?
Getting through to him in the first place is the difficulty I gather. Can you simply arrive at his home and talk to him? Does he have a family/wife? Can you tell him you would prefer to know what was happening instead of presuming the worst. That you love him no matter what. That you want to help as much as you can.
I hope this is useful to you.Please post in as often as you wish.