That's an amazingly sad story.
You can only speculate so much... This boy is 8 years old. His life revolves around himself. He's not self-aware, and he's definitely not aware of how his actions affect the people around him. That's just not in the mind of an 8 year old. They're still just trying to figure out the basics and all their social skills, never mind coping with death.
To have two people in his family die within a year of each other, people he's obviously very close to, has got to be extremely hard on an eight year old.
No one here knows where he got the idea to hang himself, whether he saw it on TV or heard of it, or saw it in a book, who knows. For him to be in so much internal pain to feel like he needed to get away from life, to be with his family, that's just heart breakingly sad.
I almost want to blame the dad for not putting the kid in therapy to help him deal with the loss of his grandfather and especially his mom. They have therapists who specialize in grievance and loss and all that, someone who he could've confided in.
I think he didn't know who he could talk to; maybe he mentioned to his family he wanted to be with his mom. His dad said he talked about her often. Maybe they didn't realize he meant literally, as in he wanted to die to be with her.
I find it very tragic that an 8 year old would feel that bad about his life, and my condolences go out to the family of the poor boy. It must be hard on the dad to lose two family members within three months of each other.