Reading your original question, several things come to mind.
First of all, both UCLA and USC are great universities. Either one will be very demanding in general, and most likely even tougher for premed students (premed classes can be quite academically challenging). There will be a lot of strong students and strong professors at either of these schools.
I wonder what the cost of attendance would be for either school. Can you afford either one with no debt at all? Can you afford either one with no debt, and still leave some money in the bank for medical school?
The third thing that I wonder about is “plan B”. Most students who start off as premed end up doing something else. Does you son know which “something else” he might potentially be interested in? If so, are either of these schools better for whatever that something else might be?
The fourth thing that I wonder about is whether your son is currently looking at any acceptances from other universities, are are other acceptances likely to show up in the near future?
And finally, does your son have any idea which might be a better fit for him?