There could be a number of reasons for the behaviour, but ultimately she should not be doing what she is. The fact that your son is scared may mean there is more going on.

Has the foster carer gone to his case manager with this information as well? It might help if both of you are speaking about the issue. It may lead to an investigation. If they are still not listening I would suggest asking to speak to a manager. If you think your son is not safe with her I would also be seeking out legal advice about whether the court order can be/needs to be reviewed.

Knowing the system too, they may actually be listening and completing an investigation, however can't release that knowledge to you unfortunately. So there is a chance that they are looking into it but I would be seeking out advise if you think your son is at risk.