Unless you regularly check comparison sites you will end up being charged more - its how the insurance companies make their money. New customers also tend to get cheaper quotes than existing ones with the intention of drawing them in and hoping they renew next year at higher rates. Nothing you can do about it but shop around next time.
In this case your Dad may be able to negotiate a discount if he tells them that he has had a look on line and found cheaper quotes - but they are under no obligation as he has agreed to the renewal.