Choose the real thing. Give him a metal table knife -- the plastic version might snap under pressure -- that he can hold comfortably in his hand. Be sure to explain that knives are dangerous and should be used only for cutting food.
Start slowly. Help him get used to the weight and feel of the knife by dabbing butter or jam on toast.
Get him in position. Have him hold the knife in his dominant hand, with his index finger on top. Now have him use his other hand to secure the food with the fork, tines curved downward.
Cut it up. Let him practice gently pressing the knife while sawing back and forth. When he's ready for cutting, start with soft foods like pancakes or spaghetti to get him in the groove before moving on to harder cuts, like meat.
Chow time! Have your child cut up all his food and then switch the fork to his main hand for eating; it'll be easier than cutting single bites and swapping hands and utensils repeatedly.