Hi mawmaw,
I'm sorry I just saw your post right now.
I am living with a bipolar/BPD woman (my partner's daughter), and it is very challenging, though she tends to be someone who "acts in" and internalizes instead of the external rages that I'm more familiar with.
People with BPD tend to have very high needs for validation, which tends to create an upside down relationship between BPD mother and child, where your daughter seeks validation from her son instead of the other way around. This could mean that he is feeling very invalidated by her.
My son's father was also bipolar/BPD and my son seems to have a sensitive genotype that made him susceptible to the emotional arousal and ADHD behaviors, among others. Until I learned to validate my son, he was on his way to mirroring his father's poor coping skills too, and like your grandson, those behaviors began to manifest at 8 years old too.
The best advice I received here and from reading books is to validate validate validate.
Here is a link on how to validate with questions:
https://bpdfamily.com/message_board/index.php?topic=273415.msg12586025#msg12586025He needs to feel that his feelings are ok, and that he is ok. In a BPD relationship, he may have some serious problems with attachment and as a loved one, you can play an important role in letting him release some steam.