Did your son get admitted to the same residential college and or academic college as your daughter? If so, I would understand him wanting to separate a bit, otherwise, UCSD is big enough that it should not be an issue at all.
Otherwise, Davis is simply amazing! We are in San Diego and my daughter is choosing Davis. She got wait listed at San Diego and although she accepted the spot on the wait list initially, now that she has visited Davis, she says she won’t accept the spot if she gets off the wait list.