Thank you for the question. It seems odd that she has been allowed to reside at his home and not her own, I could understand him not being allowed to enter if they were living together which may be what she has told the police, but there is something there that is not right unless the child was residing there. However, If the mother has mental health issues and you are worried about the child's wellbeing, I would advise the son to look at a residential order for the child. You can look at obtaining an emergency hearing by applying to court with the C2 form (you will find this online). The court may order the mother to go to a psychiatric/psychological assessment in order to determine her fitness to provide proper care for the child, if it is found she is not, the courts could grant your son, residential care for the child and even supervised contact for the mother moving forward. With the social work involvement to the degree you have stated and the fact the mother is in the son's house, there is likely more going on here but from what I have been provided I would look at going down this route.
I shall provide you with a helpful link that will assist you in finding a solicitor/representative near your local area. This will provide you with someone nearby your area who can assist you if required.
I hope this information proved helpful. You will find a local solicitor who deals with these matters on the law society webpage which is as follows;
Hopefully, I have answered your query in a way that is simple and easy to understand. If anything remains unclear, I will be more than happy to clarify it for you. In the meantime, thank you once again for using our services.
Should you require any further assistance on this matter, please do not hesitate to post a further questions for additional assistance.