Suzied, I understand your worry but, the reality is that in Henry's conscious memory, Rob isn't a relatively recent stepfather. Rob has been a significant person in Henry's life since he was three, so for most of the little boy's conscious memory. He calls Rob 'daddy' and has been asking to see him. Rob has parental responsibility.
So far, there hasn't been a comprehensive assessment of Henry's needs. Children's services were called out by the police to make sure that the emergency placement with Pat n Tony was safe. That will have involved a check of records/police and social work and a home visit to talk with the grandparents and observe Henry in their care. No concerns raised by that, so the assessment stopped there.
Rob is the victim and none of the people involved (including school and the police) have notified children's services about the unsupervised contact. I expect Pat will be on the phone as I type, demanding they "do something:
The answer is probably going to be that Rob is legally in a position to do what he has. The only way children's services can influence where Henry lives is either voluntarily (can't see Rob agreeing to H returning to Pat n TOny) or if they share parental responsibility with Helen and Rob. To apply for an ICO, children's services will need to find evidence that Henry's identified needs can't safely be met by Rob and that 'he has suffered or is at risk of suffering significant harm". Yes I can hear you all shouting but so far, no one is aware of the emotional abuse Henry has been experiencing.