These are all very good universities, it is really going to come down to fit. I do not personally have a child in any of these programs, I can only comment on my S23’s visits to SLO, UCSD, and UCD. (At the time, he was comparing programs in materials engineering.)
My son felt that he got a very good sense of these schools from attending their visit days, particularly SLO. During the open house, SLO arranged an in depth session with my son’s specific engineering department, he was able to visit labs and project spaces, and had the chance to meet and talk to faculty and students. They did a good job of structuring this visit so that he could get a detailed understanding of the curriculum and teaching methods.
Talking to students really helped my son learn about the vibe and culture at these schools, as well as how the students felt about teaching, class accessibility, rigor, support of student projects, etc… so I would suggest that your son put effort into meeting and talking to current students! UCSD and UCD visits don’t have quite as much built-in depth introducing admitted students to their specific departments, so your son may want to seek out students in his specific major program to talk to, and also contact students in any engineering clubs he might be interested in. He can look at the schools’ web sites to find out about clubs, and contact the clubs beforehand.
In general, for engineering majors, the required curriculum is going to fill all four years of study. So I also think it’s really important to look at the curriculum in depth at every school that is under consideration. Does your son feel confident that this curriculum works for his interests? Is he potentially interested in other majors (or other specific classes)? How much flexibility is built into the major?
If your son visits, talks to students, and looks at the curriculum in depth, I think he’s likely to start to see differences and get a sense of his preferences.