The English pronunciation of the s in Louis was reinforced during the 1600s and 1700s by frequent respelling of the French name Louis as Lewis. An Ngram chart for the years 1600 through 2005 shows that "King Lewis" (blue line) was the preferred spelling over "King Louis" until about 1770:
Here is a typical instance of "King Lewis" from "The Life and Reign of King Henry the Second," in Chronicle of the Kings of England: With Additions (1670):
With which words of King Lewis, the young King Henry was set a float, and from that time forward, stuck not openly to oppose his Father. Whereof his Father having intelligence, sent messengers to King Lewis, desiring him from the King their Master, to be a means to bring his Son to more moderation. But King Lewis hearing the Embassadours name their Master King, with an angry countenance said unto them ; What mean you by this to call him King, who hath passed his Kingdom over to his Son? and with this answer sent them away.
The spelling Lewis for Louis became very rare after the 1760s, but it survived until then in such forms as this (translated) title of a history by Voltaire: The Universal History & State of All Nations From the Time of Charlemain to Lewis XIV (1758).