I eventually found the following entry in the Returns of persons naturalised in 1921. Hope the link works okay.
https://www.familysearch.org/ark:/61903/3:1:3Q9M-CS9S-TN1V?i=483&cat=700375
Thank you for the link, it wasn't showing up in a search.
The name of the court is at the top of the page, so your next step would be to contact Norfolk County Superior Court, Dedham, Massachusetts, with the information on that form: his name, the date of naturalization, and, for good measure, the number in the left hand column before his name. I doubt those records are on line.
They should be "Googleable". Include Massachusetts in the search terms since I wouldn't be surprised if there is more than one Norfolk County in the US.
Try to get all his papers. There were three stages: 1) a Declaration of Intention, 2) Petition for Naturalization, 3) issuance of Certificate of Naturalization. The third is the least informative, from what I have seen, just saying that so and so was admitted as a citizen on such and such date. One of the other two might tell you the day he arrived in the US, exact DOB and place of birth, wife's name, children if any etc. They asked more information as years went by, but your man seems to have started the process in the late 1910's I would guess, so the records should have a lot of information on them.
edit to add:
He may have begun the process in a different court, but the papers in Norfolk Superior court should have a reference to them. For example, 'filed Declaration of Intention in __ Court, date of --'. Then repeat the process.