The first step in this process is to gather all his medical records, private and military, and whatever the Social Security Administration might've pulled. You, along with your mom, if available, and your dad, should review these records to understand his health issues and injuries that can be claimed. Next, connect with a reputable VSO in the area for further assistance in carrying the ball.
In case you're missing his military records, you can request them from Archives.gov, but keep in mind, that it might take a few months to get them.