Not sure how helpful this may be, but my ptsd symptoms seemed to follow my dad's death, though there had been 2 near-death experiences (my own) previously, and other stuff (sexual assault, etc). I remember as a little child being very 'aware' of everything going on, sometimes terrified, always internalized it.
Whether it was adolescence, grief, ptsd, or just myself, when the symptoms started (flashbacks, intrusive thoughts, atypical depression etc), I was 14 (I think?), it culminated (quickly) in much avoidance (of now what i realize were identifiable triggers), much rage, numbness,followed by reckless behaviour, drinking, gambling, stealing to pay for the gambling, lying to cover up all the first. Then self harm, school absentism due to being ill. Hospitalized due to what was both physical and caused by self harm no one knew about, recall that was when suicide 1st occurred to me, tried that twice. This inproved in about 4 years. It was all a desperate attempt to cope.
All the while I had a lot of anger, but felt very terrified and sad inside. Thought it was due to 'me'. Maintained straight A's (Honors, offered Mensa), worked full time, verysociable, did ++ volunteer work, no one ever knew about any of it. I was an altruistic kid and adult- genuinely enjoyed that, but did all 'the other', all the while. Never knew how to find the courage or words to ask for help, still don't know how.
I thnk it might help to let him express himself without words- painting/ pictures, etc? Watch how he plays, what he gravitates towards, what he avoids, try to be open with him.
There is a wonderful book on grief in children-will try to get the name.
Best wishes, xox