My daughter is in the same boat, had strong credentials but didn’t get in where she wanted and her top choices were UC schools (she’s waitlisted at Irvine). So she, too, is looking at community college for a year and transferring as a junior. She could TAG into Irvine or get another shot at UCLA her dream school.
My understanding is that any California community college has an edge over any other school when it comes to transferring to a UC (versus any four year school or any community college not in CA). You definitely want to encourage your son to talk to a good counselor at a CC who is familiar with the CC to UC pipeline. Some counselors are great and knowledgeable, some not so much. They’ll know what your son has to do. For example, as some people said, UCI doesn’t participate for CS but they do for Math, so if your son followed the requirements for that program he could get in as a math major with guaranteed admission and none of the heartbreak of this year.