My husband is not the bio father of my eldest child (had her in highschool). That said, he has been around since she was 4/almost 5, and we married when she was 6. She has called him dad since we got married and initiated that on her own. She loves him VERY much, and he loves her. He treats her like she is 100% his child (she knows who her bio father is, but they don't interact).
She is 12, and fast approaching that lovely, very attitude-filled stage. DH is really taking all of this very personally. He suddenly feels like her attitude is a sign of "you aren't my real dad" vs just something that almost every normal, hormonal girl goes through. I get the 'tude too, it's not just him. He grew up in a house of all boys, in a family with only male cousins, so he just doesn't get this stage.
How do I help him with this? Simply trying to explain to him that it's a normal phase that we need to work on together is not working. I'm afraid that it will eventually damage their relationship.