When I visited the UC schools with my D over the summer they basically all told us the most popular major on an application was undeclared and that this was perfectly right to select. They talked so much about it that we both got the feeling it was actually preferred. That said, my D did end up declaring a major - psychology. She has lower stats than your son and she was accepted at Davis. I’m sorry your son didn’t get in and I’m very surprised since his stats are great!
My daughter also applied to UCSD and has not heard back yet.