I'm loafing at work and came across your post.
My surname "Karlson" comes from my paternal Swedish gradparents, and was originally spelled Karlsson from "Karl's son".
Family lore says the second-born son can take his father's first name and "son" to it. The furthest back that we traced our Swedish side was to a Karl Anders. His first-born son kept the family name, and the second born son did the "-son" trick.
We "think" Ellis Island lost the second s. But my great grandfather hated Sweden, so maybe he made the change himself.
Most "-sons" I have met were Swedish and the "-sens" I have met were Norwegian.