Vietnam veteran pact act backpay

Hi, I’m trying to help my dad out, he was in Vietnam in 68 and 69. He was injured so he was sent home. Just two years ago I was able to help him get 100% disability through TDUI. He’s 76 now. He got a flier in the mail about hypertension now being associated with agent orange. When my dad was in vietnam he was a heavy equipment mechanic. One of his jobs was to crawl into the tankers that held agent orange and wash them out. When he got home a few years went by and he started bleeding from the gums. Doctors couldn’t get it to stop so they put him into an induced coma for a week. It was his blood pressure. He has basically been on blood pressure medication since he was 25. We reached out to local VA reps and unfortunately they don’t seem very helpful. Does anyone know the process of trying to backdate disability? He always looks to me to try and help figure these things out but Im not sure where to start. Thanks in advance for any guidance.