I guess one of the hardest parts of this for you is that if the kids stays with you and you split with his/her father, then you will still have to have the father in your life.
It sounds like the family will try to maybe get custody again, since you said that they had a bitter custody battle in the past.
The child wants to stay with you, but you think this is mainly because he/she thinks the dad will come back and all will be good if he waits it out.
Am I getting this right?
How old is the child?
How long has the child lived with you?
Is the child treated well by the extended family?
I'm sorry, I don't really know what to say. If your custody is only temporary, then you may most likely lose custody anyway if the dad doesn't get his s**t together?