David, this is a slightly delicate question - has your grandson got any diagnosed difficulties? The only reason I ask is I am currently experiencing serious violence from my 11 year old step son - but he has severe learning difficulties and is on the autistic spectrum.
If he has no difficulties, and mum is reaching the point where she can't cope (and in my experience, violence escalates), I am inclined to agree that getting the police involved might help. Social services may also be a good source of information and advice.