I wouldn't worry about why he is spending a lot of time with you and the girls. He may not be ready for alone time with you, but it seems he feels safe with you and the kids. This definitely sounds like a positive step forward. Don't push him for more since you run the risk of pushing him away.
It sounds like you did a good job validating some of his fears. He's afraid that you don't want him around. (Am I allowed for dinner? Spent more time with Ds than expected.) You may have never told him that he wasn't welcome, but you may have shown him through your actions.
I think that you're doing well, so chin up and put a smile on that face. You can do it!
Peace, B
Me: 29 W: 28 T: 10 M: 7 No kids 2 Dogs and 1 Cat With Parents: 09/16/07 Apartment: 10/13/07 Back Home: ~2/16/2008