Welcome to the dd/lg community here on tumblr. There is so many great Daddies and Littles here. And everyone is very helpful and friendly.
As for your question Sir, Papa, or Papa Bear are other commonly used titles I have seen. But he may not like those either. Babe, Honey, or Boo are other more everyday titles that you could use. I think it is important to first talk to him and find out why he doesn’t like being called Daddy. Some men it makes them feel old. Some men it feels to much like Sugar Daddy. And other men feel creepy when their GF calls them Daddy. Once you understand why he doesn’t like it then explain to him why you like and want to call him Daddy. He may feel different about it after he knows your feelings behind calling him Daddy. But if not maybe you guys can find a title you both agree on. Open communication is a very important part of a dd/lg.
Thanks for the question and good luck! :)