The DSS has incorrectly informed him that he isn't entitled to anything as they are treating him as a 'benefit tourist'. However, he is clearly a British citizen who lived temporarily abroad and does not need to take the Habitual Residency Test.
He needs to read all the info in this link (see below) and print it off. Most of the staff at benefit offices have never heard of the 'Swaddling Case', which is where the European Court deemed that a returning resident must be treated as if he/she had never left their home country in the first place and can claim benefits from the first date of their return.
My daughter faced this problem when she spent a couple of months with us in the USA after taking her 'A' levels and took a Gap Year....she had to 'sign on' to get a National Insurance number and to register for work. It was a battle and a lot of correspondence made as she had to appeal the decision, but once she had **printed off and mentioned the Swaddling case in her appeal** it WAS allowed and she was paid the (backdated) benefits from the first date of her claim for Income Support.
http://www.ac-company.org/en/forum_en/hrt_en.h tml
Let me re-iterate the importance of printing off this link and ensuring that the Swaddling decision is mentioned in writing if he needs to make an appeal; the Civil Servant will then need to look it up and see that his appeal must be upheld.
Hope this helps!