Did you ensure to book your father's flight in an upgradable fare class?

Only Economy class Y and B on Asiana can be upgraded using United Mileage Plus miles - these are full-fare economy tickets, i.e. the most-expensive economy fares chargeable; if you merely bought the cheapest-available ticket you almost certainly are not dealing with a ticket in Y or B class, and it therefore cannot be upgraded using miles.

See: https://www.united.com/ual/en/us/fly...de-awards.html

Note that, even if the ticket is in Y or B class, there is no guarantee that there will be a seat available in business class for a mileage upgrade.