Your dedication to Brian's life is beautiful. Its descriptions of the bullet rattling inside his brain made me think if this was a metaphor for a possible tumor? Somehow, I think I like your description better.
Brian was obviously a man of many qualities, from working on mechanical items, to offering his wisdom. I take it that Brian had a special ability to observe people, notice behaviours and accept them for who they are - who you are?
The fact his smile is with you is a gift. A gift that many wish they could have; an inner smile that can remind them that life is worth living, but only if you can make the most of it.
Well done on this, I think you actually are good at saying 'goodbye' and Brian would know this and smile.