I've dealt with this too. I was told for a long time that my short term memory was mostly a result of mental illness and head injuries, but I've been told trauma can effect it too, and that most of my issues are most likely trauma based. I do memory exercises, like repeating the letters in a spot I'm parked, or trying to remember the color of 1 specific thing and recalling that later. It took a long time but my memory is better. He would also probably benefit from therapy for the trauma, I know I did, but the therapy never helped the memory piece. That was something I had to work on by myself after researching things that helped. Looking up memory exercises for the elderly actually ended up helping me a lot (I'm not elderly, but my memory is!). Anyhow, I don't know if that's helpful, but good luck to you both!