CAS doesn’t admit by major as a freshman so it will be overall CAS admission statistics, which are also in single digits. So overall CAS is a better chance than Stern, but probably not much. But that’s just based on overall admit numbers. Your son’s ECs, essays etc will all be very important in differentiating him from thousands of other excellent applicants. If he wants to go to Stern specifically rather than just NYU, he should apply to Stern imo rather than trying to game numbers when so much else about his application matters.
What is his unweighted GPA by the way?