I'm sorry to hear about your brother. Momentum isn't necessarily make or break. Certainly when there is momentum such as in your brother's case, we see severe brain injuries. But lack of momentum shouldn't reassure people that their head bump was nothing. Natasha Richardson fell down (not into a tree). She fell on the snowy mountain and bumped her head, but was otherwise fine. I don't know how much momentum was involved because I'm not sure how fast she was going or exactly how she fell. But she didn't collide with any objects.
Here's an article that explains it in non-medical terms.
Natasha Richardson’s death and what you should know about brain injuries | MinnPost
There are multiple mechanisms by which someone can bump their head in a way that causes severe injury. The brain is surrounded by blood vessels so if you hit your head hard enough or in just the right spot where one of those vessels is affected or torn or the brain bounces around, it can cause a brain bleed. In some cases, your body will reabsorb the blood. In cases like NR's, you won't have symptoms until hours later because the bleed is slow. In still other cases, the trauma is too severe and people lose consciousness right away or don't realize there's a problem until it's too late.
In BS's case, with the amount of damage done, I can't see how a headboard injury could be the answer. A fall is much more likely, imo. But if there hadn't been that amount of damage, I buy that hitting your head on a headboard, in general, can potentially cause a brain bleed.