'Jack' has been the familiar form of 'John' for at least 800 years. Many people believe it came from the French name Jacques, but the Oxford English Dictionary - the 'bible' in matters of this sort - doubts this. They are much happier with the case - made over a century ago by the scholar EWB Nicholson at Oxford - that the word actually originated in that form as a diminutive for 'John'. In other words, it's just as much a name in its own right nowadays as 'John' or any other name is.
Your husband, Sherrardk, is 'right' in a sense, though it is more of a 'companionable alteration/familiarisation' - for want of a better phrase - than a shortening, obviously.