Parenting is not about biology, it is about love and caring. Whether or not his daughter is his biological daughter or not is completely irrelevant.
He raised his daughter over 7 years. They clearly developed the bond that fathers develop with their daughters. I raised two kids (now both adults) who weren't biologically related. I will tell you from experience that biology doesn't matter.
You are dating a man who has a daughter.
This means accepting that there is a mother involved. Many couples go through this.
The only question is this: Do you want to date a man who has a daughter?
Here's is the fact. Any time you date someone who has young children, you are coming into an existing situation. There are always these relationships, and different parenting styles, and there is often a little messiness (particularly).
As far has his relationship with his daughter, and when he will tell her that they aren't biologically related? Frankly, that is none of your business. He and his daughter will figure this out as their relationship grows. This sounds like a difficult situation... but it is a difficult situation that you have absolutely no say about.
Any time you date a person who has a child, issues are going to come up that need to be dealt with. The needs of the child comes first (of course) and there is often a history with exes and grandparents.
His relationship with his daughter, and when he decides she is ready to deal with knowing the past, is completely between him and his daughter. He has to balance what she is ready for, with what is going on with her mother and grandparents, with their own relationship.
The point being that this is not your business. You can't possibly have any valuable input.... this is an area that you haven't been a part of.
You are dating a man who has a father. If you continue dating, then you need to accept this... and you need to respect that he is responsible for his daughter and that he will be the one deciding how to best raise her. You need to accept the decisions he makes for his daughter... they aren't your decisions.
If you don't want to date a man who is a father, it is better to just end it quickly.