Finding the birthplace of your ancestor if they immigrated prior to 1900 will be more tricky. The immigration records were not as descriptive (naturalization records prior to 1900 are not too descriptive either.) Based off your information, it honestly could be either Prussia/German Empire or Congress Poland/Russian Empire. Perhaps it was close to the border? Unfortunately we don't know who gave the information to the census taker ... it could have been anyone in the house.
First, his surname may very well be Mędalski (typically the "en" = ę, which produces a similar sound.
You will want to search Piotr Mędalski in immigration records. His name would appear as Petro or Petrus in baptismal/church records.
I looked at the census records and they only indicate he was married once (so in your search bar you can help to narrow down by indicated he was single).
If you have tested your DNA, this is one of those cases where it may be able to help you. If you are able to group your matches, and figure out all the matches who share DNA via this line ... you will want to start inspecting their family trees to see if you can come up with a common region or common village. It's not that far fetched. I did it with my own line and it worked.
Also, if you know where he attended church in the United States ... see if there is someone you can contact about their records. Sometimes they have very detailed records on their members, including the village where they are from.
