Wow, I am having no luck at all on the 1920 and 1930 censuses. Very odd!
On his 1918 draft registration card, Giovanni/John listed himself as "non-declarant" - thus creating a reasonable chance that he had not yet naturalized by the time of E Mario's birth in 1926.
I think your best option at this time is to see if you can turn up naturalization papers for Giovanni. The best place to start, based on time and cost, would be the US National Archives. Go to
https://eservices.archives.gov/orderonline/ , click Order Reproductions then Immigration & Naturalization Records. When filling in the order form, you may need to put xxxxxx's or 99999's in some fields where you don't have actual info; then use the comment box for additional information. Order certified copies, as that is what you would need if you proceed with a jure sanguinis case. If the archivist is unable to locate records, you will not be charged for the search (unlike with USCIS).
National Archives requests are usually fulfilled in a matter of a few weeks. If nothing turns up there, you can try the United States Citizenship & Immigration Service. USCIS has records for all naturalizations from about September, 1906 onwards; the National Archives (for most regions) only has naturalization records from the federal court system - since a very large percentage of our ancestors naturalized in state and county courts, this leaves a lot of holes in NARA's database.
For USCIS, start at
http://www.uscis.gov/portal/site/uscis/ ... 18190aRCRD and click the "Order Online Now" link at top right. You will need to order an "Index Search" first, wait, wait, receive report, then come back and order a "Records Request" based on the results of the index search.