Where did the exact date of arrival and ship's name come from?
Was it from another website?
The reason why you may not find it on the Ancestry website (specifically on the Cephalonia ship) is because there may have been a problem with the information on the other site. I have seen this before.. It is a mis-link of some sort.
I also did a page by page search of the arrivals specifically on the 30th of April, 1887 somewhere else, and did not see any arrivals of the Cephalonia at a port of New York.
Hope this helps, as you may be looking for something that does not exist... my opinion though.
He could still be out there! You may find in arriving on another ship, year, month...
Best to get as much census information as you can on Nils Olsson. Finding census records always helps me.