Do his feces resemble the texture of dog or human feces?
I know this one is hard for you, but could he be dehydrated? Ig feces is normally part very soft semi-solid, with brown and white in it, as well as a fair amount of clear fluid.
Sorry abut my memory. Did this change of pooping happen just after you put the stuffed toy inside his cage? Iguanas are extremely territorial, and his cage (habitat) is your ig's inner sanctum. The pooping might be a response to the stress of having his new 'friend' inside his cage. But that does not mean his stuffed friend can't be good for him.
I suggest you remove it, and place it where he can't se it from his cage. He may need a slower introduction to it during his out times. He may end up carrying it with him everywhere he goes.
If you decide to take this advice, then when he's out, scrub the place where he pooped vigorously using mildly scented or best, non-scented dish soap. Use a stiff scrub brush. Do your best to remove the bathroom odor from those places in his cage.
I don't know if you use newspaper as the substrate. This would be a good time to switch to it. Lay down abut five to eight layers on the cage floor to help mask the odor. The paper also helps keeps feces from soaking down to the floor underneath. It's advantage is that it is easy to remove and replace.