If anyone needed help deciphering this code, it would help if they put it in regular text.
Remember he may have written it phonetically as he may well have been unable to spell very well. Looks like a simple Letter substitution code but with a progressing letter shift. Still, if it baffled the FBI it must have something that is a bit unique. He may well have copied it from somewhere else—or maybe he was an Idiot Savant of some sort. The Pajama Clad Freepers can break this code!