When we toured Union, the guide explained that things like Organic Chemistry would still only take two semesters and would cover the same material as in a semester, but in a shorter time. My son decided he did not like the quarter system or the school so we did not find more information. Thus, UCB, not sure the 2/3 is true. If that were the case, then 2-semester science classes should take 3 quarters to complete.
What I don’t recall is whether students take fewer classes per quarter than per semester.
OP: I hope that your son will use the quarter system as only one part of his college check list.