For some time Sweden and Norway were united under one king - but the people knew the actual name of the country they lived in and the records of the churches and administrative areas were separately kept. Norway did not lose it's identity.
For naturalization in the US one document asked new citizens to list the name of the head of the nation they were from and those US documents most often listed that as the King of Sweden and Norway... some may have reversed the word order but meant the same thing.
Your job will be to scour any and all records for Mr. Johnson AFTER he emigrated to see any ANYTHING listed a parish name where he came from. That is vital to your continued success in tracking his family history. Many times a Lutheran minister in the new country may have made some notation of the parish where a member of the congregation was originally baptized and/or confirmed -- that's one possibility.