I can see how worried you are for your grandson and am sure that you are a really good support for your son who is having such a tough time.
If you think it would help, you might like to talk with one of our people at Icarus Trust. We are a charity that supports people going through similar situations to yourself caused by the addiction of a family member. Talking through how you are feeling might help you and answer some of your questions.
You can contact Icarus Trust on help@icarustrust.org or visit our website http://www.icarustrst.org
Good luck with everything.