Praise be to Allaah.
So long as the nikaah (marriage contract) has not yet taken place, this man is still a “stranger” to you, and you should observe hijaab with him (i.e., wear proper hijaab and avoid contact) just as you would in the case of any other man. He has no right to look at you beyond the look that is allowed by sharee’ah at the time of making a marriage proposal, beyond which no further look is permitted until the nikaah is completed. What you father has told you about it being OK for you to relax the rules of hijaab in front of your fiancé is not correct, and there should be no obedience to any created being if it involves disobedience to the Creator. The mere intention of marriage does not justify uncovering in front of him. And Allaah knows best.