It may take time and patience, but with both you may be able to train your bird to "go" in the cage. First, observe your bird to see what his bathroom patterns are. Does he make certain body movements beforehand? How often does he go and what are the intervals between times? When you think it's about to happen, place him in the area you want him to learn to go. If he doesn't poop, try again in a couple of minutes. Reward him with a treat and tell him what a good bird he is in a happy voice. Be consistent and be patient.