First, he needs to be feeding off of tongs, if he isn't already.
Step one, get a piece of cardstock and color it to what you want the target's color to be. I recommend a checkered purple and white. When feeding, hold this behind the food item so he associates the color with food. Every time he takes the food, use a trigger word, such as the word "target".
Step two, after a few feedings like this (Babies take to it much faster, as they haven't had a lot of time to become accustomed to a different feeding method like adults have. I'd say about three days for yours, give or take.) Take the ball you will be using as a target, you want something about the size of a tennis ball, colored to whatever you've chosen, and hold it in front of a blank piece of cardstock. Say the trigger word, "target" and if all goes well he should nose the ball or bite it. If he does, give him his food. If he doesn't, say the trigger word a few more times. If he still doesn't, take the food and hold it against the ball. Say target, and get him to take the food. Once he does, try again without the food against the ball. Repeat until he noses or bites the ball without food.
Step three, after a week or so, try to get him to have to come after the target, instead of holding it right in front of him. Try and lure him out of the cage to get the target. Once he is consistently doing this, you have a well trained monitor.
The key is be consistent, always use the target while feeding, and don't use it to move him from A to B and then withhold food. Reward him every time he hits the target.
