PIP frozen payments

What can I do about PIP payments being frozen? My son has full PIP payments, he attended residential college paid for by the Education Authority, during the COVID 19 response the unit closed so my son came home, then was shielding until November 2021 and then kept coming home for illness and closures because of COVID. The decision maker thought that my son should not be paid when at home because of COVID because Education had paid for term-time residential college but he could not find the evidence he wanted to refuse payment. The form says dates of when you are in the placement and dates when you are home, both the college and me filled in the form with the same attendance dates. The decision maker wrote to the education authority who responded because of data protection the cannot give out specific details regarding Kris but they do not hold attendance information anyway please write to the college. The decision maker sent out the same form again to the education authority which he says they did not respond, so he has frozen all care element payments because he has not received the evidence he wants. Every time I ring I get told 'don't worry no decision has been made yet, if you are intitled to the money, we will back pay you' this has been going on for years. My son really needs that money to buy a new wheelchair. What can I do about this?