Wow not sure how to respond to this post but I'm sorry you are dealing with this.
I technically have not started my daycare yet but I was just thinking if this was happening to me I'd possibly try;
1) if child is a talker talk with child about his dad. Like ask him what he likes to do with his dad etc and like remind him that by going home with his dad he will potentially have the opportunity to do that fun thing with dad. So basically point out the good points about going home with daddy over staying with you even if you have to say hey daddy is going to bring you home so you will see mommy very soon (as the kid has no issues with mommy he will be happy to leave to go get her I'd think?)
2) mention how now that daycare is closed you really aren't that fun as you have to make supper etc so dcb will not get your full attention if he stayed and his parents would miss him if he stayed
3) I don't think I'd confront the dad at all as it does sound like he's a wounded grizzly bear just striking out at you. So definitely would try to warm the kid up ahead of time to be excited about seeing his dad or at least more willing to go with his dad home.
I definitely look forward to what others suggest as it seems like a tough situation.
Hope you don't mind hearing my two cents about it.