Thank you for posting, you are obviously very caring, and suffering yourself with the added grief of loss. It is very hard to help somebody, but not impossible. At my worst, my depression caused me to become withdrawn, I felt so helpless and worthless, I simply did not understand why people would care.
But, there is always light at the end of the tunnel, it may seem a long way off, but it is there. Would your son do things with you? Maybe a walk and a coffee somewhere? Can you engage with any of his friends? ask them for help? maybe one could call in "unexpectedly" for a coffee?
It is very difficult, I needed a lot of time on my own to begin to come to terms with what was happening to me, and that was very hard on those who care for me, especially as I felt unworthy of their care. I spent time walking the costal paths around Cornwall, finding a beach and just reading. Also, I got back to the gym after many years away, i can recommend swimming!
Keep posting, we are not a busy forum, but all want to help each other.