Have you tried contacting the Military Personnel Records Center in St. Louis?
Don't volunteer too much personal info...just that you are requesting the records of your father who served in the USMC at ________ in the 1940's. Request his enlistment and discharge info, promotion and military service records and his pension info. Include birthdate and date of death if you have them.
The address is available through the NARA.gov website. They're not that quick to respond, but very thorough in providing whatever you ask of them.
Also, have you found him in the Social Security Death Index (available online at Ancestry.com)? If he is dead and not in there, try contacting the Railroad Retirement Board on Rush St. in Chicago. (They don't always have a lot of info, but may have something that can help you...and that's only if he isn't in the SSDI).
If you think he might have been a federal employee, you can also contact the Federal Employee Record Center (also part of NARA and address is on their website). There you will need the date of birth and date of death if you have it.