Your grandpa was lucky to have lived through it. The Japanese were preparing to execute all slave prisoners of war in anticipation of the main island campaigns.[1] I think if the nukes had waited another month or two there would have been tens of thousands of allied POWs that never would have made it home.

1. Happy to lookup/provide links if requested. What immediately comes to mind is the story of Louis Zamperini which was captured in the 2014 movie "Unbroken". The book covers how close they all came to extinction (and is also much better than the movie) Also https://www.quora.com/Near-the-end-of-the-war-did-Japan-plan...