Assuming the parants are based in OZ, have not yet departed, and clearly know the routing they want, they can buy the airpasses in their names and his, and he can join them on all legs.

The DATES may be changed on most airpasses, but not the routing once locked in.

From memory they work about as low as $US60 per FLIGHT NUMBER, depending on how many you buy, whether that be SFO-LAX or SEA-MIA ..... one flight #, one coupon only needed.