I have a colleague who is a Vietnam vet, who came home with PTSD, cancer, agent orange poisoning, and host of other conditions. He had the nightmares and the rages, the headaches and the other stress conditions.
Have you been reading anything about epigenetics? It is an emerging and highly promising science that has shown the effects of severe stress and trauma on the actual functioning of our genes. That is, while it doesn't change the DNA of course, it actually changes the switching in many of the cells so that they don't function properly. This has massive implications for many autoimmune diseases as well as those who have suffered trauma. In fact, those epigenetic changes can be passed on to the person's children. There have been many studies - here's just one on the offspring of Holocaust survivors. http://www.scientificamerican.com/article/descendants-of-...
So if trauma can affect the way the cells function in children, imagine what it can do to those who actually experienced the sights, sounds, fears and legacy of combat.
My friend got tremendous results by working with epigenetic super foods and cellular nutrition to repair/reverse the epigenetic damage. He is got of all his medications (I think there were 19) prescribed at the VA. While he's not entirely "free" of those memories, and he still sits in the chair against a wall in a restaurant (as many vets do), he has his life back. He's a vibrant and cheerful person, active and healthy, cancer-free and headache-free.
I honestly don't see how tough love will make any positive impact. This is a significant mental health issue. Probably the best thing is for him to connect with other vets who have worked their way out of this hole with good support and perhaps a different nutritional approach. It's more than "eat healthy" but food science and encouragement/networking can help a lot. But I don't think it's within your power to make it happen without outside help.