Borderline personality disorder is not a result of brain injury. It is considered a personality disorder. He would have to have been diagnosed with BPD before getting the concusions to be considered true BPD.
You will get more and better information from looking up Traumatic Brain Injury (TBI), which is the result of concussions and can look and feel a lot like BPD or bipolar disorder depending on the area of the brain affected.
There are medication that help some people. I did a quick google search and there was a site that listed about 5 different meds. Anticonvulsants, antidepressants, antipsychotics, pain medication, and medication to help with movement. So if he is having ongoing issues he needs to see a specialist, likely a neurologist.
I know one person that went from almost vegetative to about 80% function, some anger and agitation when things get stressful so improvement is possible. Your BF also needs to quit doing whatever it is that is causing the multiple concussions. You only have one brain and it can only take so much abuse. Concussion essentially slams your brain against one side of the skull and forcefully push it back against the other - like dropping a bowl of jello. There are ridges inside the skull that the brain can be pushed against causing damage. The skull is not entirely smooth on the inside. Multiple concussions as they are finding out with football players can lead to suicidal behavior and anger issues. He's gotta stop doing whatever it is that is causing these concussion.
Wish I could tell you better news.
I suggest you ask around in the boderline personality disorder forum (
http://www.medhelp.org/forums/Borderline-Personality-Disorder/show/210). This is the forum on bipolar disorder, which is a mood disorder, not a personality disorder.
Other than that, I would suggest talking to his doctor about your concerns to see if they have any suggestions. And be patient! Mental health issues are just like any other health issues--they don't get better overnight. Just make sure you are not neglecting your own health in trying to help your boyfriend.