Yikes. The anecdotes are indeed a little scary. I won’t lie, my tiger mom brain is having a minor panic attack 
Sounds like if he goes the CC route the key risks are around (1) ability to enroll in the engineering classes he wants/ needs, and (2) his ability to get good grades in CC courses, regardless of quality of teaching.
I like the suggestion of re-assessing after a year in CC. The question that came to mind there was - he would only be starting CC classes this August (hasn’t been enrolled during this gap year), and he wouldn’t have even completed a semester by the time the UC freshman application deadline comes around in fall of this year. So if he doesn’t do well in CC classes and wants to apply as a freshman, the earliest he’ll be able to do this is in Oct 2025(!!) for entry in fall of 2026 (at which point he’ll be 21 years old). Am I missing something in terms of UC admissions options - eg do they take freshman in the spring?
Given admissions timelines it may be that once he gives up his current UCD admission his only choice may be the transfer route.
The best way to deal with fear is to gather info - so that’s what I’ll have him do. Get examples of other students from his local CC that did get into UCD, ask for info on course availability, understand which CC courses are potentially risky and will need extra effort or should be avoided altogether.