I have checked the register for HMOs in Eastbourne and I cannot find any reference to either a J Ridley or a K Ridley with the corresponding address. There are two properties in that street which are registered HMOs, but not no.90.
If the information you have given is accurate, and the tenancy has been in existance since Sepember 2018, it is very likely that the property is not a registered HMO. The register I checked is accurate to June 2019.
If the property is indeed not registered as an HMO, the landlord is committing an offence, the penalty for which is an unlimited fine and they may be ordered to pay the tenants compensation by way of repayment of up to 12 months rent.
So your son appears to be in a very good negotiating position, a) to get out of his contract (which the landlord is not entitled to if he/she requires to be registered as an HMO), and b) to get some of his rent back.
The alternative is that your son just reports the landlord immediately and waits for the prosecuting authority (in this case the local council - contact environmental health) takes action.
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