If he's not on a schedule, you can try tubbing him three times per day until he stops pooping in his home. First thing in the morning, the very first thing you do when you get home from work, and again just before lights-out time. The time he needs in the water is about 20 minutes. Longer tubbing is good for him though.
When he was using the tub to poop in, did he poop right away, right after you put him in the water? Virtually all iguanas do that. I had one who as soon as he saw the tub, began to defecate while I was still carrying him. Getting him into the water was a real rush job.
He might have diarrhea, with bad gut flora/fauna going on, which does cause pooping problems like this. An exotics vet can check a poop sample to look for parasites and bad bacteria, which are usually the gram-negative type. The visit may seem expensive, but the cure is easy. If this is happening and you wait, the problem will get worse. Your iguana may be in need of this vet visit, both because he is uncomfortable, and also because of possible malnutrition. I hope you can afford the occasional vet visit, because nearly all captive iguanas need vet care from time to time.
We don't have all the answers about pooping problems.