Yes I can relate. I was extremely depressed to the point that I would not talk to anyone, wouldn't eat, laid on the couch from morning to night just looking up. I only got up to use the bathroom and feed my dog.
No one could help me because I just tuned them out. It didn't matter what anyone said. I had to find it in myself to realize I needed help. That was really hard for me to deal with because I didn't understand why I felt so sad. Nothing bad had happened so there was no reason for me to feel the way I did.
I don't know why your son feels the way he does. All I can suggest is don't give up on him.
Good luck and I hope everything turns out okay.