Obviously your now ex-boyfriend did not make a clear cut break-up if he still texts you and still wants to go out to dinner with you. Seems to be the same pattern he has with his past ex-girlfriend. They obviously never had a clear cut break either! And now they are both jeopardizing the posibility of her child having a wholesome happy family.
I don't quite understand that your best friend thinks your ex is worth fighting for from what you just wrote about your ex.
If you are still meeting him for dinner on the weekend I think you should show strength and make that clear cut before your order and then leave. Only this way you will be able to grieve the loss of the good things you had, so you can move on. Don't put yourself in the position of being second best/ on the reserve bench/ the seat warmer etc.