His chances will be dependent upon how many accepted students enroll by May 1 which can vary from year to year.
UCSB 2019 Waitlist stats:
Waitlist Offers: 8,863
Waitlist Opt-Ins: 5,865
Waitlist Admits: 604
Waitlist data for UCSD 2019:
Waitlist Offers: 20,000
Waitlist Admits: 4,300
No guarantees if admitted off the waitlist, that he will get CS as his major.