A brief thought on this...
God has a plan for all of us, including the children of A's. Just as it isn't your job to save the A, it's not your job, or even in your power, to save the child either. The life he will live will get him to where he needs to be in the future.
If you can stay in his life, and be a spot of sunlight through the clouds for him, than great. But losing sleep and serenity over what's going to happen to him is not good for you, or him.
Find some readings on detachment maybe?
Good luck, I hope this trip to rehab works out. Very nice of the parents to help clean up his mess before he goes in, I guess? At least you don't have to do it.
My childhood wasn't perfect (ACOA), but it made me who I am today. I wouldn't change it for the world.