The 121st was not a part of 103rd ID, Tamie. My WWII reference says 121st was inducted into federal service 16 Sept 1940 and assigned to 30th ID, relieved from 30th ID 22 Nov 1941 and assigned to 8th ID at that time.
His serial number begins with a 3, which indicates he was a draftee. My guess would be that following induction and training he was first assigned to 103rd ID, then was transferred to 121st Infantry, then shipped out to Europe with the regiment. (Please remember all this is entirely speculation.) His date of death (15 July 1944) would coincide with the 8th ID entry into combat (12 July 1944). Other pertinent info is the fact that the 103rd ID did not arrive in Europe until 20 Oct 1944.