There are a few ways to do this, and usually it is at least a little stressful (except showing them to a female or using natural sunlight.) I would consider it a good thing that your animal is comfortable enough that it stays a very calm color. If you do want to try to get him to fire up, natural sunlight, showing him his reflection, showing him another male, or (sometimes) showing him something very bright/colorful (flowers, chameleon toys, brightly colored clothes, etc.) will often work. Some will also seem to get very upset at seemingly random things (one of mine would only fire up when he saw my guinea pigs.) Keep in mind that any of these things, save showing him a female or using natural sunlight, should all be very brief experiences.