Nasty. Following the letter of what he promised, but not the spirit. It would never have occurred to either Philip or the Queen to stipulate a hereditary dukedom.

There, I kept my promise. I've made you a duke, but I never promised anything about any sons you might have. Unlike my sons, your son doesn't deserve to be a duke.

Nasty, petty, and shabby. And yet another example of how he carefully parses his words and never clarifies them when people assume they mean "X', when he's always intended them to mean 'Y'.