The most successful 1 year transfers have a combination of AP/IB and DE credit taken while in HS. AP credit alone makes the 1 year transfer and TAG difficult since the applicant has no grades to submit or a minimal number of grades with only summer session grades/units completed at time of the TAG submission and Fall grades are not reported until the TAU (Transfer Academic Update) in January. Also he needs to be meet the 30 semester/45 quarter units completed by end of Summer for TAG.
TAG GPA will be calculated based on the Community college courses only since AP course grades are not considered in the UC Transfer GPA.
An applicant can TAG to one of the 6 UC campuses but can also apply to the rest of the UC campuses as a regular transfer.
Your son can apply to a CC during Senior year of HS as a backup and just not attend if he gets admitted to his choice UC.
He should use assist.org and figure out a tentative schedule starting with Summer session to see if this plan is feasible to complete for TAG submission and matriculation by Spring 2027.