If UCD really is his first choice, he should say so and, essentially, commit to going (morally).
Have him think ralistically: if he got into UCSB and UCD, which one would he prefer?
If not, wait till he hears back. He could also call Admissions and say UCD is in his top 2-3 choices and, indeed, send his 1st semester grades + any update + ways in which he’d contribute to college life, along with the appeal.
I don’t think indicating interest in other majors would help at this point, since it wouldn’t really seem “genuine” and perhaps perceived as an attempt to game the system.
I agree that 4.19 UC GPA for MechE didn’t make UCD a safety.