Since he was in the Marines his WWII Service Record was not stored at the NARA when they had a fire in 1973, so it should e available. As a cousin I don't think you qualify as a next of kin able to get a copy but you can try or have one of his siblings or children (if he had any) order a copy.
But another mystery.. The "U.S. Rosters of World War II Dead, 1939-1945" is an Army Quatermaster database so John Steve Stevens remains must have been recovered by the Army in 1943 or 1949 when he was moved to HI burial. Contact the Quatermaster Corp or look at the NARA for a record of WWII burial codes. http://www.archives.gov/publications/ref-info-papers/82/part-1.html#rg024http://www.archives.gov/publications/ref-info-papers/82/part-1.html#rg024