Personally I would not take such chance because legally your son's entry did not meet the eligibility. That you were not challenged better count your lucky stars. Should you get a stingy officer, the outcome would be quite different and might be very unpleasant.
Better play it safe than be sorry.
PS
Now I remember you are from Australia! In this case, your son holds an Australian Passport - then of course he can ENTER as Visa on Arrival, with maximum 15 days stay. That is WHY there is a VISA Stamp on the passport because IT IS A VISA on Arrival.
It is COMPLETELY DIFFERENT from TWOV because you son's entry is NOT under TWOV, but Visa on Arrival granted to New Zealand, Australia when it was announced in Mid 2024. Now it seems the days of stay is extended to 30 days.
Those who hold other passports better check the Chinese policy of their nationalities. US passport DEFINITELY CAN NOT do the same as your son. Period.
