I would wait too. Growing up I never had a 'dad'. my bio dad died when I was really little, and I had 3 step dad's after that. The first one made us call him pa. We hated it and him! The second was a major loser but we just called him by his name. The third one is awesome and my mom couldn't be happier with him. But I still resent him some I guess. He came around when I was 13....just bad timing. But, if anyone has ever been a dad, it is him. I will never call him dad though.
Basically just because your kids call him by his name, doesn't mean he won't end up being dad if you guys stay really serious. The person makes the dad IMO and in your case it sure sounds like E is a better dad than their bio dad. One day your kids will see that too. But even if you guys are together forever and ever and they never call him dad, no harm done because they will still know that he is the best dad they ever had.