Contact the vetrans administration. They will have his records. If he is dead you have the right to his medical records for your own history. You can also use genealogy as a reason. You can get this info under the freedom of information act. You just might have to deal with their BS for a few months. If he is alive they would forward a letter from you to him and it would be up to him to contact you. Don't know if they still do this.
I was able to get my father's medical records that way. Also grandparents names. They give you a run around but eventually you get somewhere. In my case he was deceased and I had to send a copy of his death certificate. I waited too long to search for him.
Keep records of all correspondence and good luck to you!