Trying to trace Grandfather, trouble was he was possibly in OSS

Hi

So my Dad found out that there was a secret about his birth. His cousin had been investigating the family tree when she discovered that there was something odd about dates not matching up. Namely my Dad was born in 1945 which is impressive as my Granda was still out in North Africa. Through probing the two found out the truth and discovered that Dads real Dad was in the American army and had claimed to be in the Secret Service. My Dad has now died as his his cousin. I’d love to find out more about his history but I’m sure will come up against issues if he was in the secret service. I live Northern Ireland in the UK. My Granny was a nurse in London. Any help?