Prescription charges: another ESA to UC problem

My son has just been sent a prescription by GP. He used to get free prescriptions on ESA but now, while waiting for Universal Credit, he is not entitled to free prescriptions. I thought we could pay and ask for refund receipt called FP57 but the chemist is telling me that he cannot have that form and should wait until he gets Universal Credit or pay. That doesn’t sound right.

Anyway, once I receive the statement or the first payment from Universal Credit does that mean my son will get free prescriptions again ?