Using was stopped, Steinbeck depicts a state which may not be permanent — perhaps the grandfather might take another breath — not a completed action from which there could be no recovery. That way, the reader determines that the grandfather is indeed dead at the same time as Casy. It's all about point of view. What it isn't all about is the passive voice.
If I can say
His breathing was labored.
Then I can say
His breath was stopped.
The past participle is to be parsed as an adjective would be. This construction is no more in the passive voice than
The car was stopped at a red light.
as opposed to a real passive
The car was stopped by the police.
But in a very short time his breath was stopped. The other three crept on their hands and knees, till two got to the shaft and were drawn up; but one of them died in a few minutes. — The Journal of John Wesley, 1952.
Until that moment I had never known what it was to be so afraid my breath was stopped, my body functionless, myself incapable of doing anything but stare in helpless shock. — Richard Matheson, A Stir of Echoes, 2013.
Her breath was stopped up in her lungs. It took effort to haul in oxygen. Her hand dipped in her purse again... — Kylie Brant, Waking Nightmare, 2009.
The patient swells up and nearly suffocated, feels as though his breath was stopped in the middle of his throat. — David Grove, Tapeworms, Lice, and Prions, 2013.
Then the king blessed the bread, and bade him eat it, and the morsel abode in his throat and choked him, so that his breath was stopped, and so died wretchedly. — The Golden Legend, Vol. 6.
In the first and last examples, death comes after a person's breath was stopped; in the others, a person only feels like death is nigh, but the state of not breathing was only temporary. In none of these sentences is the verb in the passive voice.