Step parenting

Guys need some advice. My finance and me have been friends before i got together with him. He was always involved in my daughters life (from a previous relationship) she is 2 and very intelligent but he now has told me he's not happy he really struggling to take on the stepfather role and feels like my child hates him because she knows its not her biological dad. How can i make him feel better. He is amazing with her and she does love him. Any advice or things i should do?😔