TBI changes *everything*.
Much of what you describe, he cannot possibly “help”.
His brain has been rewired by the injury and this may be his new normal.
How and what he “thinks” is being influenced by damaged neural pathways, some of which can totally change a person to “someone else”.
There are therapists who specialize in TBI and you should try to help him find one.
It will not ‘fix’ the TBI side effects but it can help him to understand what has happened to him, why he perceives things as he does, now, and hopefully he can find ways to cope with/manage it.
TBI is a living Hell I would not wish on anyone.
/TBI, too
I have to respectfully disagree.
There are numerous examples of those with brain trauma being successfully helped with or the effects of years of stress and/or depression (another form of trauma), to state nothing of the complicating factors of drug (legal or otherwise) and alcohol abuse.
https://www.rewireme.com/brain-insight/5-myths-ptsd/
The relatively-new field of study is referred to as Neuroplasticity.
2 conflicting references and a 3rd supporting recovery:
http://healingtraumacenter.com/neuroplasticity-and-rewiring-the-brain/
https://www.psychologytoday.com/us/blog/workings-well-being/201703/how-heal-the-traumatized-brain
For the OP, this is pertinent to starting the healing journey, but I concede that you have an uphill battle to get your son to admit some small steps can change how he feels:
https://www.jennifersweeton.com/tools/