If your child is in foster care, it's part of case plan development to do a family assessment. The goal of the state is to eventually reintegrate the child with the parents. If that is not possible, then parental rights are terminated and the child becomes a ward of the state. But termination of parental rights is not something that is taken lightly. The state has to show that they have made every effort to reunite the child and the parents, but that cirumstances prevent that from being a possibility. Legislation sets out what circumstances warrent the termination of parental rights, and the placement of a child in foster care. So the state is just doing their due diligence when it comes to your son's case.
What would probably be best would be to talk to your caseworker about your concerns, and how to get your son some treatment. You don't have to let them walk through your home. But your unwillingness to do so will be noted in the assessment that goes to the judge.