That's sounds so heartbreaking, and I'm so glad you've been there for him and cared for him. He has finally learned to trust you so much that he came into your house when the winter came. He's really one lucky guy.

I never heard of a kitten disease like that, would be interesting to learn more about that.
While reading, I thought he might have eaten some poison, because some toxic substances can cause symptoms like that. Or he was injured.
But you're right, you'll probably never know... However, he sounds like a great fighter!

Kirsten