It really shouldn't be this hard to find words, right? However, finding the right words, even with all the languages I do know is not easy. So, the letter ends up being rather short, but the implications carry my intent. And, I'm sure he's smart enough to get them.
Kurama,
Arigatou gozaimasu
M
((OOC: The implications are "This means, literally, 'It is difficult to be.' We can interpret this more loosely as, 'Such kindness (as you have shown me_ is rare,' or in more fanciful words, 'It is difficult for me to continue to exist after having been overwhelmed by such kindness.'" --
Easy Japanese: A guide to spoken and written Japanese by Jack Sweard c1992, page 31))