I am sorry to hear about the difficulties with your grandson. I can understand your concern that this has caused. As you can imagine, it is very difficult to make a diagnosis without having met your grandson, so I wouldn't want to give you any false impressions. Especially when drugs are concerned, it is difficult for a clinician to make a diagnosis when a patient is on drugs as they can affect the person's behaviour significantly and it is difficult to establish if certain behaviors and changes are due to mental health or due to drugs. Someone would also need to know of any history of mental health of the child and the family. The behaviors you described could be symptoms of many disorders, firstly drug abuse but also manic depression, psychotic episode, conduct disorder, or personality disorders. However, it could also be a combination of depression and drug abuse. As I said, when someone is on drugs it is very difficult to establish any diagnosis. In all cases, your grandson wold need to get help in a specialist clinic firstly to address his drug addiction. If he managed to quit the drugs and still the problems continued then his parents would definitely need to consider psychotherapy. This could be done either 1:1 with your grandson and a therapist or if the family has certain communication issues (as you said above) then it would be suggested for the whole family to engage in family therapy. In all cases, therapists do not take on substance addicted patients and they require them to be clean before they commence therapy. So tackling his addiction would definitely be a priority.
You could share these thoughts with his family and discuss it with them. In the mean time I am afraid that there is not much more that you can do apart form being there for them and supporting them. I am sure that your support is invaluable at this difficult time for them.
I hope this helps
All the best