I'm not usually a grave dancer but
The U.S. should take advantage of any possible disorganization or cracks in loyalty to his son to effect change. The first thing I'd do is cut off the food supply. We feed 2/3 of North Korea's population, even though they didn't hold up their end of the bargain (to stop building nukes).
Cut off the chuck wagon and see how long it will take before the populace throws him out or kills him.