Thanks for your question.
If the property is genuinely uninhabitable then this would be a repudiatory breach of contract by the landlord. This is a breach which is so serious that it goes to the heart (ie. The root) of the contract so as to deny you the whole or substantially the whole of the benefit of the contract.
The issue with terminating on these grounds is proof. If you can prove on the balance of probabilities that the property was uninhabitable then you will have a defence to any claim made by the landlord for rents under the tenancy.
You need documentary evidence. So this is photos of the issue and correspondence from you to the landlord (with proof of posting/emailing) stating that you consider this to be a serious breach of contract which makes the property uninhabitable.
The ideal form of evidence would be in the form of a declaration of uninhabitability by the environmental health department of the local authority. You should speak to them and ask for an inspection. If they attend then make a declaration then you have good evidence of inhabitability and could rely on this, the photos and your correspondence to the landlord as evidence to terminate.
Your son should write to the landlord stating that the problem still persists and that they require it rectified otherwise they will consider it a repudiatory breach and terminate the tenancy.
Depending on what the environmental officer says, the photos your son can get and whether the issue is solved by the landlord following further prompting he may be able to terminate on the basis of inhabitability
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