Who was my grandad ?

My granddad ENGVALD (NILBERG) was born 29. september 1889. His mother was Marie Iversen and his father unknown (actually his father was a shipowner in Trondheim, Norway whom Marie Iversen worked for. We don't know his name). At the age of 15 (approx 1904-05) he was sent (by his father) to America to get an education. He started his own business over there and was (or so I have been told) moderately wealthy. He was a member of Nordmanns-Forbundet and left America just before WW1. It was meant to be a short visit to marry my grandmother Emilie Lillevold Johansen, but as the big war started she did not want to go. They stayed in Norway and later moved to Strømmen, Norway, where they stayed untiol my grandfathers death at by cancer 42 years old. I've been searching the web for information about his life in America. We're having problems because we don't know if he took the name Nilberg before or after leaving Norway for America. My aunt thought his fathers name was Johan and that he might have used the name Johansen. OR the his mothers name Iversen. Search in Norwefian databases has not helped to find any records of his birth or of his mother Marie. We wonder if he started a family in America before he went back to Norway and that this might be the reason why he did not return ..... Any ideas where we can go from here ? Kind regards Elisabeth Nilberg