Just my $0.02, hope you don't mind. I was just thinking that his family may even visit this site and read this thread, which may kind of detract from the original intention of the letter which is hopefully to express a spontaneous, heartfelt condolence from the heart. The letter, if you don't mind me saying, is absolutely fine as it is, and its intention and spirit will be greatly appreciated by Billy's family. I just know from prior experience of writing notes of condolence myself, that it's all too easy to sit for hours wondering whether this word or that phrase will have the right effect or be taken the wrong way, or that you've said enough. I had this recently when the child of a Facebook friend died. I sat there with my fingers hovering over the keys, typing things and then deleting them, when in the end I just wrote the simplest of condolences. At times like this it really is the thought that counts.
As a total newcomer to this forum I didn't know Billy at all and had no interaction with him, so really this has little to do with me, but I tell ya, having gone to the guy's apartment building to inquire about him and hearing the super say "I'm sorry sir, he passed away 4 weeks ago" through the intercom really brought home the human element of it and choked me up to be honest, so I've been following the tributes to the guy with interest. I think it's great what you're doing and believe me, whatever wording the letter takes, if it's basically along the lines of how it is now, is going to be absolutely fine.