I've been reading through all your posts, through the ups and downs, and was so hoping to hear that he was improving. You really fought hard for him, and I'm so sorry he gave up the battle. I know your heart is broken right now, but just know you did what you could for him, and he will now be at peace.
I hope that you get to find out what happened to him. Poor little guy, it'll hurt now, but atleast you know you did everything you could for him! May he rest in peace!