- Mon père ne se sent pas bien. Il a mal au dos.
Sesh seems to have gotten her words mixed, dos means back but it is only used to talk about the human back. Therefore he can not be "back" you can say il a mal au dos, meaning his back hurts.
The answer is "est". Former part of the sentence means, My father doesn't feel good. "au dos" in the next portion means "back", So it should be he is back, there you've to use être and we know the "il" form of être is "est". So the sentence will be, il est au dos, meaning He is back.