I know nothing about your particular person, but it is NOT true that discharge records will always show the complete truth of a serviceman's assigments during war or otherwise.
They may show where he served and in what capacity, in what component of the service he was in, but IF he was involved in some sort of special ops, that info may NOT be in the normal discharge papers you receive.
Some records will not be declassified for a period of 75 years from the date the event took place, some never will be.
If you believe what this man told you, then search to find the truth.
Contact members of the outfit in which he served to see if anyone there knows and will tell you what they know.
Try getting in touch via one of the reunion or alumni groups attached to his unit. Also contact your congressman or woman and enlist his or her aid in finding this information, if you truly believe it is so.