Mandatory recon and DWP response to appeal

My 30 yr old son has severe epilepsy, sadly considered permanent and unlikely to improve by his consultant. The original decision said his award was for 18 months because surgery might improve things, without any evidence at all to support this claim. After another consultants letter expressing his surprise at the decision and reiterating that my sons condition is permanent, the mandatory reconsideration told us the length of the award had been increased to 3 years, and the letter stated they were unable to increase the term of the award because my son has a limited right to remain in the uk. This is utter nonsense. After we lodged our appeal, we received a reply from the dwp in their submission to the Tribunal which says the statement about my sons right to remain was an administrative error, and giving a new, different reason for limiting the time period of my sons claim to 3 years, ie that his medication could become more effective. Again, with no evidence to support this claim. Are they allowed to move the goal posts like this? Surely our appeal is against the mandatory consideration reason, not the response to our appeal new reason? How can they base their decision on unfounded guesswork, in contravention of our consultants and our gp’s evidence? The tribunal is on Friday next week so I need help as soon as possible, please. Thank you in advance. HH