The Witness of the Sun by Suki Fleet

Ilya knows little about the world Before. All his village has left are pictures in books of glittering cities made of glass, of dangerous creatures like bears and wolves that used to haunt more than the forests. Ilya knows little of the wars that forced people to flee the cities and hide.
One dusk, when Ilya and his sister are hunting rabbits, a bear, a creature that they were told no longer exists, comes into the glade, and everything Ilya thought he knew about his world begins to disintegrate. Especially everything he thought he knew about Caleb, the mysterious boy none of the villagers can understand. Getting to know Caleb feels secret. But Ilya has too many secrets of his own—he can feel heartbeats, sense the forest, the warmth of all life within it. And Ilya has not always lived in the village…though where he comes from, he does not know…