Was your son employed full time when he took out the PPI and if so was he self employed? Was he over 18?
I think really we also need a copy of the loan agreement to see exactly what was in it. It's highly likely that the loan itself is enforceable as it was only taken out 4 years ago and most loan agreement then would have all the prescribed terms required by the Consumer Credit Act, but it would be useful to see the wording of the PPI statement as well as the terms and conditions. Often you are not told that you can buy PPI elsewhere.
You can request a copy of the loan agreement by sending what is known as a CCA request (template N
here ). Enclose a £1 cheque/postal order with it and send recorded delivery.
Obviously the PPI needs to be cancelled ASAP as it will still be mounting up. So if you haven't already cancelled it you need to make sure that that is done.