My grandpa's military history is confidential?

When my grandpa died in 1991 they did the 21 gun salute...gave my grandma a flag and also some medals.... my grandpa was a marine... my dad asked at the funeral why he had received some of the medals and they said that all information is confidential and could not be released for 50 years. MY grandpa was in the Korean war yet his name does not appear in the korean war books.... he also came home from korea later than all other soldiers but he is not listed in the POW or MIA archives??? He also changed his name before going to Korea from a polish name to a german name (he was polish)??? WHere can i find out more information on his military past?