My mother was never allowed to read them and then they disappeared.
A stranger told us they were selling their home and found boxes of these.
Her letters were of course blown up when their ship suffered a near fatal blow in the Solomons, but what we have from him is absolutely tear-jerking.
It's just some 18-year-old kid writing about how he hates the Navy and wants to get home to his love so she can "fulfill her promise".
Then, after the attack, he is all-grown-up and says "I have seen the first battle and I hope to see the last".
My 2 cents.
Donate the letters (or copies) to a local museum or historical institution(s) if you do not want them or if you think you can't care for them. Every story is unique and can not be replaced - the historical significance of his writings depends on the context(s) they will be placed in one day. Ultimately we tend to see the past as it is presented in the present and someone might pick up his story in the near or distant future. We can not know for sure if the story will ever meet a (numerous) audience, but that is really besides the point if the letters won't be properly preserved in the first place. I would argue that this is especially important in a time, where the numbers of surivors of WWII are declining and where some people tend to forget the lessons the war.
Just check (beforehand) if the museum or institution does not have some bad or weird reputation.
And btw. Sounds like a real interesting story to me and I can imagen a lot of veterans (old and modern a like) can relate to this story one way or another. Sounds like your grandpa was a real badass.
Good luck.
Definitely make a digital copy and either keep it as a family heirloom or offer it to a museum/historical society. My grandfather was conscripted by the germans in '44, but he was quickly captured by americans (probably that is what he wanted) and I have his diary from the time, mostly letters he wrote to my grandma.
Check the love letter archives in Koblenz (Germany), a digitisation project for - well- love letters ... It's a cooperation between University of Koblenz and Technical University of Darmstadt. They'd love to digitise them and make them publicly available. PM me, if you're interested, I can introduce you to a team member ...
Edit them into a self-published Amazon book. My wife's family has a Jewish guy who escaped Nazi Germany to the US with family, and signed up to fight Hitler. He was a prolific letter writer and went through hell, combat, fatigue, loneliness, liberating some camps. It was turned into a compelling historical document with a little contextual editing between the many letters he wrote home. Spoiler -- he lived, had a big family and died at a ripe old age.
More can always be said, but this older answer might be of interest for you.