The following link should take you to the VA's regulations that covers the issue you that you are working on. They refer to it as 'personal trauma'. It is in section 17.
http://www.warms.vba.va.gov/admin21/m21_1/mr/part4/subptii/ch01/ch01_secd.docAre you working with a veteran advocate or veteran representative? There are ways of providing evidence to support the claim. As you know, most incidents go unreported but there are signs of changes in the victim's duty performance and actions.
Another thing you may want to keep in mind and that is to not lock in on just PTSD. If there were signs that he suffered from depression and/or anxiety during service then he may want to file for PTSD/anxiety/depression or any other condition that could had been caused by his service time. It doesn't matter what the VA wants to call it. It all pays at the same rate. Too many veterans give the VA an easy way to deny a claim by using the wrong wording.
It is hard to change direction in the middle of a claim but this may be helpful if the VA denies the claim for PTSD. I hope this information will be of help to you.