The Life and Time of Lonny Quicke by Kirsty Applebaum

Lonny is a lifeling. He has the power to heal any living creature and bring it back from the dead. But he pays a price for this gift - by lengthening the creature's life, he shortens his own.

So Lonny has to be careful, has to stay hidden in the forest. Because if people knew what he could do, Lonny would be left with no life at all... A brilliant novel!