I have no idea if this will work or not with an iguana, but a long time ago we had a pet rabbit in the house and we potty trained it by scooping up it's poop and dumping it into a litter box. Once it saw/smelled it's poop in there, it used that box from there on it. So maybe try gathering up his poop and dumping it in the cage.
No, the iguana's poop, not your son's. That would be messy. ;)