Can't seem to potty train my Argentine tegu

I have a 10 month old Argentine tegu and he's developed a very bad habit of pooping on the couch. Whenever I take him out to play we typically go to the couch because it's easier to handle him there. But recently he's become very comfortable with pooping on it, which is a total pain to clean up. Every time he does it I punish him right away. But I can't tell if punishment works with tegu training, or if he's learning from it. I would hate to think he just thinks I'm being mean, and I hate punishing him because he's SO CUTE. So I guess my question is twofold: Can tegus learn to stop behaviors from punishing them when they do it, and also how do I keep my tegu from pooping outside his cage? I know they are smart enough to learn and really don't want to deal with tegu poops on my couch once he is full-grown :P
Thanks!