I agree, you need to do a satisfactory job training this guy. My other strong recommendation is to start looking for a job elsewhere, as it is obvious the company you are working for does not realize your true value. This way, you preserve your reputation, such as it is, with your former employer, while taking advantage of the market while there is still time.

Realize that eventually they will probably dick this guy as well, so it's not like training him is going to do him any favors in the long run, only help you get rid of a problem and allow you to move on.

If you think you can get away with it, you might not tell this guy everything you know. Give him enough to take care of the basics, but keep a couple critical details just to yourself. That way when you do leave for greener pastures, you will be missed, but not blamed.

I did this on a recent assignment, and was able to negotiate a substantial increase in my pay and reputation in the market, while at the same time putting my previous supervisor and the new hire in a bit of a bind. I worked it in such a way that his peers realized how much I did that I wasn't getting credit for, and why would he have let me get away from them. Now I use my former dept manager as a reference instead of the supervisor that passed me over, and he likely won't go any higher for a long time.