Keith, let me recommend (VERY highly!) two books -- you read them, and see if it helps you 'see' PTSD, and then, as you come to know your 'new' son, see if he might like to look them over.
This first -- amazing amazing amazing! Waking the Tiger: Healing Trauma
and the second -- even more amazing: In an Unspoken Voice: How the Body Releases Trauma and Restores Goodness
There are a couple of YouTubes of Peter working with vets -- very moving/motivating. If YOU can come to see how the trauma is 'held' in the body, perhaps you can help your son see it -- and 'finish' it, which lets it go.
Waking the Tiger offers a new and hopeful vision of trauma. It views the human animal as a unique being, endowed with an instinctual capacity to heal as well as an intellectual spirit to harness this innate capacity. It asks and answers an intriguing question - why are animals in the wild, though threatened routinely, rarely traumatized? By understanding the dynamics that make wild animals virtually immune to traumatic symptoms, the mystery of human trauma is revealed. Waking the Tiger normalizes the symptoms of trauma and the steps needed to heal them.
I hate to ... bastardize ... my deeper understanding of his work, but I haven't time to write out a thesis! (I made Chico read it too -- I'm sure he can say he benefited.)
Levine starts with where and how he 'got' the idea; (his own car crash incident) and connected some AMAZING dots -- (again, I'm shorthanding and "bastardizing" -- read the whole thing!) If you look at the antelope who is chased by the lion... if he is lucky enough to get away, then he returns to the rest of the herd and goes back to eating, but he keeps, for example, shaking his skin and twitching and stomping a foot... This is PART of the (normal mammalian) recovery from a near-death experience.
We humans, on experiencing a near-death experience (or what feels like one) metaphorically run screaming from the PHYSIOLOGICAL after effects. We DON'T "shake it off" -- let the body burn through the adrenaline and other shock/fear/escape chemicals that enable mammals to escape and live -- and of course, for young guys pinned down by a sniper or pinned in place by an IED -- EVERY SINGLE CHEMICAL response in their body (PHYSIOLOGICAL, not specifically -- or rather -- separate FROM the mental/emotional!) says "run for your life!!" But, instead, our young (or old) guy has to lie exactly still, calculate where the sniper is, hold down his (entirely normal!) physiological responses, and hold his rifle still and steady to shoot back. He does not GET to 'finish' his BODY'S "run for your life" chemical storm. And, when the young guy gets back to 'his herd' -- when he SURVIVES the attack by the lion -- he forces down the physical responses that are the 'finishing' of a mammal having just escaped death.
Having NOT finished, the body keeps trying to bring it up to clear it, and we human choose to drug it so as to NOT let the body deal with the shaking and crying and twitching and general 'clearing' of the chemicals; or push it down harder (till it explodes: outwardly or towards self!).
So, when the guy gets back, and he finally can no longer 'hold it in' -- even if he GOES to the VA and a shrink -- all too often, the shrink wants him to rehearse and replay the 'bad' incident -- to re-release the shock/fear/escape chemicals he hasn't been able to finish before -- and then he's given "techniques" to NOT DEAL!
We moderns have a really BAD habit of assuming "it's all in our heads!" (<-- CHICO!! What do I keep yelling at you?!). Our BODIES are screaming for relief and help, and we stupid big-brained humans try to TALK ourselves out of it! (Imagine if you have Type 1 Diabetes -- can you TALK your pancreas into functioning again?!)
Anyway -- Levine is brilliant and I LOVE his stuff!! Recommend it very very highly! (He IS working with the VA, last I heard -- but he's one old guy, and there are many thousands of PTSD young folks!)