What do we do when we can't help them anymore?

As some will remember, I've had worries about my son for quite a long while. He's 44, and over the years has taken recreational drugs. He's now really depressed and has enormous feelings of guilt for how this has affected family due to his lying etc. I'm just about at my wit's end, now, as I don't know how to help him. He's not been going to work, so I'm scared he'll lose his job. His flat was really messy when I saw him the other day, and now he's not answering his phone or my texts.
I realise I may be adding to the pressure by trying to contact him, but I just need that bit of contact.
If he loses his job, he'll lose his flat, and he'll not receive any benefits due to being dismissed. He's got a GP appointment for the end of next week, but I'm fearful it's not soon enough.
I feel a bit better for putting this down. We just can't mend everything, can we, however much we try?