(Extra luck if he plots against you, since then it's just a -10 malus.) Then his son becomes the heir. You can imprison and execute your own son if you're willing to take the major malus. (There's no apparent cap to the long-reign bonus, or there wasn't back then, so in the last 20 years of his life he could basically do whatever he wanted.) Because of the succession laws, when he died he had outlived all his children and his oldest grandchildren, but his oldest son's oldest son's oldest son (his great grandson) inherited the throne at the age of six or so.
My longest-reigning character inherited when he was only a few months old (his father died of wounds sustained in battle a few months after his heir was born) and ruled until he was 89. A run of bad luck means old heirs, but much of the time you'll wind up with a young, but adult heir.