Pro-rata

Hi there
Can anyone please advise me on pro-rata? My husband works for a scaffolding company here in malta, and on the 6th and 7th august he had 2 days holiday with pay, and then had the following week off without pay, as he wants to keep his holidays due for a trip in november. All was agreed with the boss, but when he recieved his wages at the end of the month, he had been stopped an extra 2 lots of national insurance hence he actually paid 3 times. All his boss said yes thats right its pro-rata, and doesnt want to know, this surely isnt right, he said you have to keep up your national insurance payments. So basically by having a week off it has cost my husband money, so after many arguments he his still none the wiser, so was wondering if anyone knows anything about this sort of thing, or even point us in the right direction to someone who might. Our understanding, was that you pay tax and insurance on the amount you earn per month, hence it could go up or down according to hours you have worked, Would be very grateful if any one can shed any light on this issue,
Many Thanks
Gaily