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Hello, this is Jim, a dual-qualified lawyer (UK & Republic of Ireland) and happy to help you today.
There has been a case of misrepresentation here - had your Son known then he could not have signed.
Powers are available to Local Authorities under the Environmental Protection Act 1990 if the property is considered a threat to public health. Rat infestations are a serious matter so your Son should terminate the contract, demand his money back (if the landlord refuses, send a letter before action such as the attached, give them 30 days to pay and if they do not then start a claim for your losses at http://www.moneyclaim.gov.uk). In the letter, threaten a report to the local council to report the landlord for this.
I hope this helps? If I have answered the question would you kindly leave me a 5 star rating (at the top of your screen), any follow up questions are welcomed.
Yes, I will ask customer services to do so for you, thanks