This is an issue that has filled hundreds of pages of debate in this forum. There are many component pieces to the issue, so it's not easy to summarize it briefly. However, I will try...
The KJV was essentially a re-write of previous English editions (all from the 1500's), which depended on the Masoretic Hebrew manuscripts from about the tenth century AD, and on Erasmus' five printed Greek editions of the New Testament. Wycliffe's translation (late 1300's) was not a primary source, though I'm sure the translators were familiar with it. There are issues with both Old and New Testament sources; Old, because most of the quotations of OT passages in the NT don't match the Masoretic text wording; and New, because Erasmus had less than a dozen manuscripts available to examine, none of them complete.
In the centuries since 1611, there have been nearly 6,000 Greek manuscripts discovered, and thousands in other languages. Also, the Septuagint Old Testament (copied from Hebrew into Greek between about 250 and 50 BC) has come to light, along with other major discoveries such as the Dead Sea scrolls. Simply put, there is far more (and much older) source material today than in 1611, and there is greater understanding of both Greek and Hebrew.
There are charges of corruption (inadvertent and intentional) against many of the source materials
not used for the KJV. These are difficult to substantiate, and the arguments often sound more like conspiracy theories than sound research-based evidence. At the same time, there is a greater
volume of evidence for the text underlying the KJV. Sadly though, some people have become so focused on defending the KJV that they have crossed the line into idolatry, even making claims of divine inspiration of its English wording.
For those who have become familiar with its language, the KJV is often the preferred translation; that's fine, as long as those who prefer it don't compare everything else to it, because it is merely one translation, and not the standard against which all translations are to be compared.
For a more thorough and scholarly treatment of the subject, I highly recommend James White's
The King James Only Controversy. Perhaps after you read it, your grandfather will be willing to do so.