With tubbing, he can't climb out. He stays until he poops, or until 1/2 hour goes by. When he poops right away, his reward is removal and a nice free-roam session, or back to his hab if he wants that. When he stays for 1/2 hour, his reward is a nice session of free roaming if he wants it. He will for sure get used to the tub, and he's likely to learn to love it. I converted many igs to tub-popping this exact way. I thought it was very convenient for me.
This has worked for many igs in the past for many participants here. Are you reluctant to take up another duty to help him out? Start tubbing him as I described, for crying out loud. It may take a while, but he'll straighten himself out. And he is well able to associate the act and the subsequent reward. He just isn't as obvious and quick to learn as a Golden Shepard.
You picked him as a pet. Start the training, Dude.
Don't worry, we like you because you like iguanas :-)