OK, I'm just gonna come out and say this, whether it be right or wrong;
This isn't an actual Zodiac letter. Rather, it's a piece of artwork INSPIRED by the Zodiac letters. Want proof? He signed his name of the bottom! This, of course, means it probably was never meant to be solved in the first place. Still, it's pretty darn good fascimile, so hats off!
However, I'm willing to rescind this comment if it is truly a Zodiac letter, and he merely got his name from the letter.
Sorry if this ruins everyone else's fun.