Backdated PIP Payments

In March my son's application for PIP benefit was sent to DWP.
In May he had his PIP assessment at an assessment centre.
In July my son had an operation on the Tumour in his knee.
Beginning Augast my son's PIP claim was denied.
Early Augast I appealed the decision.
By mid-September, my son's condition had improved. I informed the DWP of the improvement from Sept. I then recived notice that my sons appeal had again been denied.
I am now taking my son's case to the tribunal.

I need to know if my son has a right to the back dated PIP from the date he made the application in March untill September when the DWP were notified of his improvement, which would greatly help as it would mean 6 months of backdated payments to him?