ME is probably the most popular Engineering major at all the UC’s, so it is just a huge amount of competition with a record number of applicants. My definition of a safety is that you are comfortably above the 75th percentile in test scores and GPA, with that said, I would have put UC Davis at a Match not a safety school. UCD also admits Engineering students by Department while many other UC’s admit first into the University and then into the specific college, so this can make a difference on the acceptances. Add in essays and EC’s to the application review and you will have more variability in the subjective portion of the application. Having gone through UC admissions with 2 son’s and my niece, all I can say is many times it is a crapshoot.
In regards to applying to a less competitive major, many times this can backfire since it can be very difficult to switch majors later on, so I do agree that your strategy was sound. I used the same strategy with my younger son for CS, but his stats were good but not great. We tailored his college list to very realistic choices so he had several options in the end that he was more than happy to attend.
For appealing a rejection, he would need new and compelling information to make UCD consider taking a 2nd look. Any notable awards? Competitions? Significant increase in grades?
Where he goes for undergrad will not define him but what he does with opportunities given to him.
Good luck to him and I am sure he will do very well where ever he ends up this Fall.