Time is allowed to pay for the parking, or nobody could ever park legally ! However the time taken must be reasonable in the circumstances, and here it all looks reasonable. So your son should submit representations that when he arrived, he had to first download the app needed to pay for the parking, then actually make the payment. His parking session was only one minute after the PCN time, so they should give way on this, but probably won't as all they can see is ££££ signs in front of them. So you son may have to forego the discount, and take them all the way to adjudication. It is this or just cough-up, I'm afraid.
Q: Is your son the name and address on the V5C Registration Certificate for the vehicle ? A Notice to Owner, the next stage in the process is sent to that name and address.