Hello hela,
I'm sorry to read about your relationship difficulties. It sounds to me from just reading your post that it's your former boyfriend who needs to be finding out more about his condition and why he's feeling so badly. His personality changes sound really extreme and concerning.
A lot of those PCS symptoms you mentioned (light sensitivity, concentration issues, dizziness, pronounced anxiety, etc.) do often get better with time and rest and sometimes treatments, but not always. Some can be left with residual problems permanently even though they may not be as severe as immediately post injury.
The fact that it was a second impact injury makes things more complicated as well.
I'll post the link to the TBI/PCS forum below in case you ever want to check out the posts there. There is a wealth of information on that forum, both in the threads and also in the "Sticky" posts at the top of the forum that contain useful websites, resources and posts on hope and healing. You will find posts there also regarding personality changes after TBI.
Traumatic Brain Injury & Post Concussion Syndrome Forum
It sounds, from just reading your post, that he's lost and needing help. (p.s. when I say needing help, I don't mean from you. He needs professional help from those familiar with these types of injuries. There are great support groups too in the US (if that is where you are) for those injured and also for those who are caregivers). I'm very sorry about your relationship and you've obviously tried very hard to make things work.