qom, until barmaid comes along, here in the UK, it kind of works the same way in that paid holiday entitlement depends on the number of hours that you work and on having worked them within the year. If you only work half the hours of a full time job then you are only entitled to half of the holiday.
Employers are usually generous and allow people to take paid holiday in the expectation that they will work enough hours in the year to cover the holiday, its only an issue in the year that you leave. If you leave before you have worked enough hours in the year to cover the holiday that you have taken then the firm are entitled to take the appropriate amount out of your final salary payment. they may not do so, but unless it says something else in your contract, then they can. What does your dad's contract say about it? It sounds like the firm are saying that he has to accrue leave by working before he can take it.
Actually when my DH was in the merchant navy (oil tankers but it worked the same everywhere) it worked the same way. When you were working, you accrued leave at the set rate and then you took it when you left the ship. When you had used up your paid leave, then you could go off pay or go back to sea. Sometimes the company would want you back on the ship before you leave was up and you either could save the leave or sell it back to the company.