AN A320 CREW RESPONDED TO A GPWS WARNING DSNDING INTO TUC.

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I WAS PNF WHILE APCHING TUS. I ADVISED THE FO (PF) THAT HE NEEDED TO STAY HIGH TO AVOID GPWS WARNINGS DUE TO HIGH TERRAIN W THROUGH NW OF THE ARPT. AFTER HAVING BRIEFED HIM HE CONTINUED TO DSND EVEN BELOW THE GS FOR THE ILS RWY 11L. WE WERE NOT ON THE LOC COURSE BUT WERE APCHING THE LOC S OF THE COURSE. THE ARPT WAS CLRLY IN SIGHT. AS WE CROSS A RIDGE; APCH CTLR ADVISED THAT THEY WERE SWITCHING RWYS TO RWY 29R. WHILE I WAS TALKING TO APCH AND ACKNOWLEDGING THE CLRNC THE FO CONTINUED TO DSND AND SHORTLY AFTER ACCEPTING THE NEW RWY ASSIGNMENT THE GPWS WARNING ANNUNCIATED. WE EXECUTED THE PULL UP MANEUVER AND ENTERED A L DOWNWIND FOR RWY 29R. THE FO DID NOT EXECUTE AN IMMEDIATE GAR AND I INITIATED THE INITIAL GAR; THEN RETURNED THE ACFT TO HIM. THE AUTO THRUST WAS NOT RESPONDING TO COMMANDS AND I HAD TO TELL HIM TO USE MANUAL THRUST UNTIL I COULD REENGAGE THE AUTO THRUST. THE FO WAS A RESERVE WHO HAS NOT BEEN FLYING MUCH. HIS NORMAL PROCS PERFORMANCE WAS EXCELLENT BUT THE NON NORMAL OPS WERE WEAK. FOR MY PART I SHOULD HAVE BEEN MORE INSISTENT ON HIM MAINTAINING BETTER ALT AWARENESS. AS PIC I TRY NOT TO INTERVENE WHILE THE FO IS FLYING BUT REVIEW THE PROCEDURAL ERRORS AFTER THE FLT. THIS IS ONE SIT IN WHICH I SHOULD HAVE INTERVENED AND BEEN MUCH MORE ASSERTIVE ABOUT HIS ALT CTL.