“Let him mourn for a few days, then gently encourage him to get excited about his real options.”
Agree with this. I predict after he bonds a bit with his new school he will be fine. Half way through his freshman year it will be “UCLA who? What was I thinking?”
I went to grad school at UC Davis, and there was a sizable subset there among the undergrads who were disappointed not to have gotten into Berkeley. They often came to campus with big plans to transfer to Berkeley as soon as they could. I never knew even one of them to later actually attempt to transfer. They grew to love UCD so much and so fast they forgot all about Berkeley.