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Google Ngrams says into his pillow is more common than onto his pillow.
Example sentence:
He sank back into his pillow with a groggy groan.
My first thought was that you can only be on your pillow, not in your pillow. But now I'm not sure.
Why is into more common in this case?
Onto portrays the pillow as a surface.
Into makes sense if the pillow is fluffy and partially surrounds your head. Assuming an ordinary-sized pillow, "He sank into the pillow" is another way of saying (more literally) "His head sank into the pillow".
I'm pretty sure sank is the reason here, it becomes a metaphor in which into is used since "sank" literally proves a rhetorical point.
The word into refers to the person slowly sinking "into" the pillow and not actually/literally the head being inside of the pillow. You can say it as in the head sank into the pillow because of its fluffiness or softness in which it can't hold the head like other pillows, which are a little harder; indicating that the head kind of compresses the pillow due to its weight.
Onto would have been the correct word to use if, for example, the sentence was like this.
He groaned, jumped onto his bed, hugged a pillow and slept.
Good question! It’s not something I consciously think about often, but into and onto are very tricky words: I remember one fellow from Mexico, who had grown up using en for both and finally thought he had it: onto means on top of, into means inside. Then he’s at the airport, and the loudspeaker announces that everyone should get onto the airplane. (I suspect what’s going on there is that we get onto ships, trains and passenger jets because those have large decks that we walk on, and into automobiles, small boats and the cockpits of small airplanes because those have compartments we sit in, but I don’t really know why my ancestors decided it should be that way.)
Personally (I’m American, and it might be different elsewhere), something going on or onto the pillow would make me think that the pillow is supporting it or holding it up, and something going into the pillow would make me think that the pillow is enclosing or absorbing it.
So, I would “rest my head on the pillow” or “elevate my injured leg on pillows.” I would also sit on a pillow or place an object, perhaps a piece of chocolate or a rose, on or onto a pillow. I would put a pillowcase either on, onto, around or over a pillow, and pull a sheet onto or over it.
In this context, he sank into his pillow because “sank” means that the pillow is too soft to support his head, so the soft pillow now surrounds his head. His head is, in a sense, partly inside the pillow. He might also carry something either in a pillow (if he wraps the pillow around it) or on a pillow (if it is on top of a pillow that supports it). He might cry into his pillow, because the pillow absorbs his tears, or scream into a pillow that muffles the sound.
If your head, or some portion of it, is currently located in some space that would otherwise be occupied by the pillow, but for your head displacing it, then your head has gone into the pillow.
