Ancestry.com has the U.S. Marine Corps Muster Rolls for 1798-1940. I searched the records and found a Steven Jandrokovic who enlisted on April 3, 1923 and served in 18th Co., 2nd Battalion, 5th Regt, 4th Brigade, MCEF, in Sharpsburg, Maryland. I suggest you write to the National Archives to obtain his service record (see instructions at http://www.archives.gov/veterans/military-service-records/get-service-records.html
The 1920 census lists Stephen [sic] as the son of John and Jose (?) Jandrokovic (both born in Croatia) in South Brownsville, Fayette County, PA.
I hope this information helps.