Found

There is no first time, not really, because time doesn't pass in the linear way mortals like to think it does. But if there were a first time it would be this, Shiro singing him awake as he takes a break in his travels, sitting against the broad moss-covered trunk of a tree.

Keith has no true sense of time passing; he'd sunk into a sleep here in the deep dark of the woods when most of these trees were just saplings, and slumbered as they grew into forest titans around him.

Shiro's song breaks through his sleep, and Keith rises, taking on a human form for what feels like the first time. He feels the pull, and he's helpless to resist it. When Shiro sees him and smiles, Keith knows this man is worth waking for.   He's worth anything.