Have you checked the CEL website for what evidential photos they have of the vehicle they say is the one restored to your son?
As
@jfollows says, do not enter into any communication with a useless debt collector. They are powerless to do anything except make the low-hanging fruit on the gullible tree pay up out of ignorance and fear.
As you say that the vehicle could not be your son's van, you need to find out if the operator has misread the VRM of the vehicle that was actually parked at the location. Either that or your son's van has been cloned.
Go to the CEL website as though to appeal or pay (don't actually do either) and check their photographic evidence. If it was an ANPR photo, there should be images of that on the Notice to Keeper (NtK). Could there be a mistake with the VRM? Sometimes a holding screw can make a letter or number be misread by the ANPR.
If that is the case as I suspect, then you have a very valid case for pursuing a claim for a breach of the Data Protection Act 2018 by CEL and you should also make a formal complaint to the DVLA.
So, instead of panicking about useless letters from a powerless debt collector like DCBL, answer the questions and, if possible, show us the original NtK (not any reminders).
READ THIS FIRST - Private Parking Charges Forum guide