He is made one with Nature: there is heard His voice in all her music, from the moan Of thunder, to the song of night's sweet bird; He is a presence to be felt and known In darkness and in light, from herb and stone, Spreading itself where'er that Power may move Which has withdrawn his being to its own; Which wields the world with never-wearied love, Sustains it from beneath, and kindles it above.
The youth is now one with nature, says the speaker.
His voice is present in thunder and the sounds of birds. You can feel him in the darkness and in the light, and everywhere along the earth.
Nature (which the speaker also calls the "Power") drew Adonais-Keats back into itself, but wasn't selfish with the youth. Nature shares him with the world, which helps keep the world going.
The speaker isn't being literal here; he just feels that Adonais-Keats' spirit isn't gone, even if his body is.