The story is about a neolithic shaman who becomes injured by aliens, and when they heal them they make him immortal.
Pretty sure this is the novel In the Face of My Enemy by Joseph Delaney. Below is a book review and cover:
Eighteen thousand years ago a priest ascended a high place, there to hold converse with his gods. That night the gods chose to bless him with their physical reality. Alas, mortal flesh cannot bear the presence of godhead, and so he died. And was reborn. But ever after he was not as other men. He could change his form at will, his wounds healed instantly. . . and he did not age. As millennia passed he grew very wise, and came at last to understand. His destiny lay not among men, but among the stars.