Speaking from experience, the elders will do nothing if you send them a letter. They believe in corporal punishment and if a father carries it too far, well, that's his god-given right. The elders treat it the same way they do sexual abuse: they'll do anything to cover it up so as to protect the 'reputation' of the Organization.
I would really start to bug Child Services. Tell them that if something happens to your grandson, you'll make sure that it's exposed that they had been warned for years about the abuse, and that they sat back and nothing.
Keep us posted.