Looking for any help

Hi everyone I am looking for specific information or directions to reasearch sites for my Father and Grandparents. My Father was born in Oslo Norway in 1940 he came back to the us via the Stavangerfjord His sur name at birth was Sivertsen but somewhere along the road his father changed it to Severson...I am trying to track down any info about thier travel, re entering the US, how the name was changed, any resources that might help me get more information. Thanks so much! Kirstin