i think the situation is this:-
when you paid off the original debt you must have given your debit card number. This, in effect, is tacit consent for them to take any payments of a recurring nature. However, you then spoke to them and said "take my card details off your system". At this point one of two things have happened:-
1) Son has given YOUR card details to repay them. (UNlikely, i think since i imagine his MO is to run up debt and the not face his responsibilities). If this IS the case, it is fraud - they will undoubtedly look to your bank to refund you.
2) They have simply decided to use your card details to reclaim their cash despite the withdrawal of consent. this being the case, this is an unauthorised payment and THEY should be refunding it.
My money is on number 2 happening. Your son is not dishonest, he's just stupid. IF I am right, you can tell them to take a running jump.
Smow - collect as much detail as you can, dates, phone calls (check bank statements phone records etc), write as much as you can from memory. Get it all compiled at this stage in case it goes **** up.