Aww, I'm sorry about losing your little guy. I know how hard it is, especially when you can't work out what's wrong. Just rest assured that you did all you could, and gave him the best possible life.
I'm so sorry to hear about your little guy... Sometimes there just isn't anything you can do about these things. Something kind of similar happened to my Puck... I asked about it here on the forum and nobody could come up with anything. At least he had a good home and was well loved during his lifetime.