A few points:
You say the property was 'unlivable' - but did your brother actually stop living there - i.e. did he move out - with all his stuff - and incur accommodation costs elsewhere? Was it wholly unlivable or just partly so?
Did your brother incur any other
provable costs
as a direct consequence of the 'unlivable' condition?
Got all the receipts to prove the costs incurred?
See
http://www.housing-ombudsman.org.uk/ and read their factsheets. They can't help if the landlord is not part of their scheme - but you may glean some info - and their phone number.
Avoid courts and lawyers if you can. They eat well - so don't come cheap! You and your brother are at risk of having all costs awarded against you. See if the landlord will agree to
and accept as binding some cheap form of 'alternative dispute resolution' - e.g. an arbitrator. Maybe the ombudsman or cab can suggest someone trained in dispute resolution.
Does your brother have an 'alternative agenda'? (E.G. 'All landlords are nasty, I'll make this one pay.') Press your brother to settle fairly, quickly and without rancour - and be seen to do so - and have documentation to show this. Remind him that his future landlords will take up references and, if he gets a bad one, he might just need to show that he was more than reasonable in this dispute.
Try talking to the landlord sweetly and reasonably. Remember your objective is to get this dispute resolved.
'Ring-fence' the problem - i.e. make sure all amounts that are not in dispute have been paid or at least payment offered.
Good luck!