Pax, by Sara Pennypacker, was published in 2016 by Balzer + Bray.
Pax and Peter have been inseparable ever since Peter rescued him as a kit. But one day the unimaginable happens: Peter’s dad enlists in the military and makes him return the fox to the wild. At his grandfather’s house three hundred miles away from home, Peter knows he isn’t where he should be—with Pax. He strikes out on his own despite the encroaching war, spurred by love, loyalty, and grief, to be reunited with his fox. Meanwhile Pax, steadfastly waiting for his boy, embarks on adventures and discoveries of his own.