Since he was writing coded notes since he was a boy, I'm surprised that they couldn't find one of the old notes that perhaps a family member could attach context and therefore some meaning to. Then, with a base established, they could build off of that.
Then again, it could be a red herring. The notes could be meaningless symbols from a mentally ill mind. It's common for mentally ill hearing people to speak in meaningless jumbles of words, and mentally ill deaf people do the same thing with real signs in wrong usage, or totally made up signs. That's something interpreters working in mental health settings have to be aware of.
I wish the copy of the note was clearer. Even the version on the link wasn't clear enough. (Right now I'm not home and using my baby netbook, so maybe that's limiting my view.)
If it is a real code and someone cracks it, that would be interesting.
It reminds me of the Navajo Code Talkers during WWII. The enemy could never crack their "code" because it wasn't really a code in the usual sense. It was their native language, mixed in with a few specific code names for military terms.