You make a valid point about the car not being sold on finance.
The reality (I know, yes, it's not legal) is that many people sell on these financed cars through ignorance of the law. The finance company really won't care/find out as long as your son uses the proceeds to make the payments. I've come across many cases in commercial disputes (more than a few involving 'little old ladies') where the car has been sold on, but never a case where the finance company has tried to recover the car when payments were being made.