Have your cadet check with his Cadre for Cadet Command Circular 601-17-1 if it was published. The older CC Circular does include the Tricare 708 option for newly commissioned cadets (not their dependents) for after commissioning and prior to accession on AD. There are eligibility criteria... like no other insurance.
Here is an older post on this same topic.
https://www.serviceacademyforums.com/index.php?threads/tricare-eligibility-question.42870/page-2 Thanks
@-Bull- The NDAA 2004 first put this into effect.
@MohawkArmyROTC may have some insight too.
You could check on the Tricare website. I have found Tricare 708 on their website previously.
For our family it was free to retain the newly commissioned officer on our family insurance plan because it was the same rate for one child as all children.