SDP - “the law” says they can only backdate 13 months?

It have recently discovered that my son was entitled to Severe Disability Premium from April 2016.  At that time he had a social worker who helped him move into his own flat with supported living.  I had never heard of the benefit and the social worker seems to have overlooked it.

I applied to DWP on his behalf and received a payment covering 12 months backdating.  However I have asked them to backdate to April 2016 but they say “the law” says they can only backdate 13 months if it is your fault that you didn’t claim regardless of the circumstances.

I think this is very unfair as my son is completely incapable of dealing with financial/benefit matters and clearly with his mental health problems he is at a disadvantage.  If his social worker wasn’t aware how on earth is he expected to fathom it.

I have tried to look for a legal authority for their decision but cannot find anything at all.  I called them today to ask for more detail and they said the decision had been made in accordance with the Social Security Act 1998 s109 and Social Security & Child Support Decisions and Appeals Regs 1999 regs 7(2)(b) and 8(3)(b)(1)

I’ve done a google search but can’t find S109 and the latter two sections make no sense to me at all.

Can anyone shed any light on this please?  I may well be just being very thick. ??

Thank you in anticipation.