Even if there isn't much you can do, it sometimes helps to unburden when you're among others who care and try to offer comfort.
If I were to offer any advice, it would be for you to remain a stabilizing force in his life (Something he can hang on to).
Whenever possible, do things with him (Assuming you live near him), even seemingly trivial ones that resemble a quiet normal life. Pick up a deli sandwich and go to the park (Good place to talk), go for a ride, or maybe ask him to help you plant some plants. He needs to re-learn what it's like to live normally.
If he makes bad choices, try not to become an enabler. Excessive drinking or self-medicating never solves anything, it just adds another problem to the one you already have. Try to help him see that there is hope, and a way forward. In the end, all you can do is your best, and the rest will be up to him.
Take care of yourself as well. It's very distressing to watch your child in trouble and feel powerless, but try not to dwell on the negatives. Hopefully he feels comfortable talking with you and sharing his emotions and what he is going through. You want to keep that communication open. There are no guarantees because a lot is up to him, but you never know what little element can turn things around. It may be something as simple as seeing an old photo that reminds him of a happier time in life, and he may want that back again.