How big is your ig's water dish? Most igs prefer to poop in water. Try putting a larger dish of water in the cage - preferably one that your ig can fit it's entire body in and get in and out of easily. You might be surprised how fast he starts using it. I've heard bigger igs do well with cat-litter boxes - but fill it with water, NOT cat litter!!!
Another method of training is tub-pooping. For me and Xander, tub pooping reinforced the frequency that he pooped in his water dish when forced to go in his cage. He never poops in his water anymore. Tub pooping is easy. You just fill up the bathtub with warm water (about 88F) to their chest level and let them soak. Don't take him out until he poops. The water should stimulate him to poop pretty quickly, though. If the water starts to get cold, add more warm water to it.
I've found that if I tub poop Xander (who is about a year and a half old) first thing in the morning, at noon, and in the early evening, he doesn't poop in his cage at all. I don't know if this is feasible for you or not, though. The thing about tub pooping is that you have to be consistent. Once or twice a month my husband and I will have to leave and be gone all day. During those times, Xander simply poops in his water dish.
Hope that helped. If you plan on letting your ig free roam in the future, this can be handy to keep him from pooping on the carpte *most* of the time (accidents DO happen!)
Alika