Sounds as if he is a loving/involved/completely available father, and you want to explore the possibility of something more serious. Clearly, it is easier for him if things are casual because no one is unhappy.
It may be that you can accept a casual relationship with him for now (while remaining open to the possibility of meeting someone else if this man ultimately cannot be more serious with you)?
Or perhaps it is simply the wrong time for him to have a relationship due to his family priorities and commitments.
I suspect he has such a strong commitment to his dc, and has submerged his own needs for so long that he can't/won't give himself 'permission' to have any sort of life of his own - especially as he is likely to get push-back.
I am going to go against the grain here, and say that if the two of you were/are serious, there would be nothing wrong with him saying to his dd something like 'I'd be happy to take you and your friends to the theme park but I can't do it this weekend as I have made plans. Let's choose another time that suits all of us.'
As I said before, this would only come with a more serious relationship and would be tricky at first because everyone is accustomed to him being completely available all the time. They'll be unhappy, and he is likely to feel conflicted/guilty. It would be easier for him to simply continue as they always have done.
Obviously it goes without saying, he'd be with his dc if they had plans, if there was an emergency, big school event, etc.
Does his ex have a new partner?