The saying goes, "If you've seen one brain injury, you've seen one brain injury."
Every injury is different. It is best to identify the individual symptoms and treat, work around or accommodate them individual.
There are personality changes that can be temporary or long term. One common change is called rigidity of thought. This can manifest as stubbornness or difficult letting go of a single concept. The denial can be this rigidity of thought. Trying to push through it will be counterproductive. The stress of the conflict caused the brain to turn to fight or flight mode causing the rigidity to increase.
One of the problems with mTBI/PCS is the long term changes. He can continue to develop worse symptoms for 2 or 3 months. By then, it is easy to attribute them to something other than the mTBI.
A challenge he faces is even recognizing his symptoms. It is not necessarily denial but rather ignorance. If he has been self-directed and self-reliant prior to the injury, it is almost like he needs to fail miserably so that he stops to try to figure out what is going wrong.
So, the best you can do is study up on mTBI. The TBI Survival Guide is you best resource. Find it at www.tbiguide.com
. You can download the 84 pages to print out. Then, when he has a struggle in front of you, you can excuse the behavior as a result of his brain injury. Maybe a " That is just your brain injury acting that way." or some other careful but appropriate comment.
Be ready to explain how you come to this conclusion. If he opens the door to reading the TBI Guide, make the most of the opportunity. Let him learn at his own speed. It can be quite humbling to accept the dysfunctions and changes to a person. As you commented, It can be like the grief process, just the initiating of the process can be delayed and full of conflict.
I assume he is driving. If not, or if he is but his driving is suspect, this can be an opening to some testing and therapy. A neuro rehab clinic should have the means to test his abilities. A follow up with his doctor may be able to start this process.
As you can see, there is no simple answer. Go slow. Get as much knowledge as you can.
My best to you.