My Son Is Carrying His 8 Week Old Baby With

My son is carrying his 8 week old baby with his head dangling over his arm. I am concerned he is not supporting the head and neck. I have commented but he says he like to arch his back and likes it. I am having trouble finding an article to support my concern. I think it is because everybody know to carry an infant supporting the head and neack and so no one tells you the possible injuries. They only mention the brain injury from shaking syndrome from severe shaking. I am not worried about that but feel the way he is carrying his baby around seems dangerous.