He does have TRICARE coverage on his own once he graduates and commissions, prior to reporting for AD. It is known as TRICARE 708. Basically he can use his orders and CAC card to access healthcare for himself
Tricare went through a major restructuring last year, and I see no mention of the Tricare 708 plan anywhere other than here and on Reddit. All posts are before the Tricare restructuring.
Tricare offers Tricare Young Adult for 23+ adults, and of course he will be eligible for Tricare Prime at this first Duty Station.
SA cadets get Tricare Prime, but the Tricare website conspicuously makes no mention of ROTC graduates.
This is something I'd strongly suggest asking his cadre and/or calling Humana Military (for Tricare East) directly and asking.
A LOT changed last year, and a lot of plans were discontinued. I wouldn't count on any info older than ~Jan 2018.