A young man..perhaps 14 or 15 years old.
Trøgstad is south-east of Oslo, close to the Swedish border.
Wilhelm B. Dahl left Norway April 29 (!) 1870, together with parents Ole Halvorsen Dahl (Carpenter) and Johanne Wilhelmsdatter, children Augusta (7), Wilhelm (5) and Torvald.
In 1880 they live in New Prairie, Hope, Minnesota. Additional siblings Tilda, Johny and Hilmar.
I think Anonymous has the right man. I'm not sure how/why the photo from Coon Prairie, Wisconsin... However Coon Praire is one of the earliest and most famous Norwegian settlements in the Midwest.