My father gets mad when I don't say I love you it's just hard for me when he's got other family that says it he's my birth father?

this is something that you should let the truth answer. Do you still love him? If you don't, there's no point in saying so. And if it's because he spends more time with his other family, you tell him that.

You say he's your birth father but don't give any details about your relationship. Does this mean you were raised by other people?

If so, then it's natural for them to be the ones you have bonded with. Being in the room when you were conceived does not entitle anyone to your love. Maybe if you are still getting to know him it's too early for any demonstration of affection,or maybe there are issues between you. Either way, if you're not ready to say those words,you're not ready-nobody can force you,he needs to accept that.

you might just be shy, explain to him about that, maybe he'll understand

sounds like you have some unresolved issues between the two of you that you both have to talk about