The employer certainly has a duty of care to employees. There should be specific instructions in the residents' care plans re dealing with aggression and assaults. Not enough to say "it's part of the job" - ok, yes it is, but there have to be safeguards. I hope your son is not dealing with matters on his own - it sounds like there should be a high staff/residents ratio. Whether the organisation is a local authority one, or private, or charity, it has to be inspected by the Care Quality Commission (this replaced the other inspection bodies). If you google their website, it'll give you info. This situation shouldn't continue. Has your son been given training? Do senior staff re-assess things?