Bear with us while we discuss this behind the scenes - we want you to win just as much as you do but we want to find the right balance between maximising your claim without risking too much in court fees, and in possible court costs awarded to the defendant bank.
Tell your son and think on this. He can pay the £160 and have no further worries from them. If he read POFA Scedule 4 he would find out that if he went to Court and lost which is unlikely on two counts at least  they don't do Court and 2] they know they would lose in Court] the most he would be liable to pay them is £100 or whatever the amount on the sign says. He is not liable for the admin charges as that only applies to the driver-perhaps.If he kept his nerve, he would find out that he does not owe them a penny and that applies to the driver as well.
But we do need to see the signage at the entrance to the car park and around the car park as well as any T&Cs on the payment meter if there is one. He alone has to work out whether it is worth taking a few photographs to help avoid paying a single penny to these crooks as well as receiving letters threatening him with Court , bailiffs etc trying to scare him into paying money he does not owe. They know they cannot take him to Court. They know he does not owe them a penny. But they are hoping he does not know so he pays them.
If he does decide to pay, tell him to wait as eventually as a last throw of the dice they play Mister Nice Guy and offer a reduction.
Great. Whatever he pays them it will be far more than he owes as their original PCN is worthless. Read other threads where our members have been ticketed for not having a permit. [We know so little about the situation that we do not know if he has a permit and forgot to display it. ]