SDP backdate on closed ESA claim?

Hi,

I am trying to help someone who was advised to move to UC for help with housing costs.
He was in receipt of ESA IR and PIP Standard daily living. Had been living alone this past year but before then lived with his Dad who was on AA.
He was not aware he needed to inform ESA IR that he was now getting PIP- he thinks this was awarded last year.
I was advising him to contact ESA and advise that he is on PIP to get SDP put on claim, THEN move to UC.
He called ESA who advised that his claim is closed as he made the claim to UC on 4/12/23.
He does have some issues with remembering etc.

He is in his 2week ESA run on period, so while he cannot go back onto ESA, is the claim actually closed? Could they look at the SDP retrospectively? I would have liked him to have the transitional amount added onto UC if possible.

thanks