How to get a replacement birth certificate if born abroad on army base?

My Dad is in his 50’s and about a year ago his old roommate had kind of breakdown and threw out everything of my Dad’s while he was on a trip, including some documents and his birth certificate. He was born in Germany on an army base, he has a German birth certificate and a US one (he still has German one, the guy threw out the US one). His Dad who was in the army passed about two years ago. He still has his social security card, it’s in very bad shape and he tried to replace it but they refused because he didn’t have a US birth certificate (the German one doesnt say anything about the army base). Now he is about to renew his license, and needs his birth certificate for that and not sure what to do as he tried and they told him he is not a legal citizen (false! They came back to US when he was 3 months old with his US citizen birth certificate). He doesn’t have a passport. We’ve tried looking online to see if we can order one, but we’ve only seen websites for US state births. He is not computer savvy and works 24/7 so I’m trying to help him find a solution. What the heck do we do?