Green Guy does this too. When I get up in the morning the first thing I do is fill the tub. He sleeps with me, and when he hears the water running (and sometimes even before I start the water) he waddles into the bathroom on his own, and often climbs into the tub on his own. He soaks for a while, then poops and gets himself out. While he is soaking, I clean his house and make his breakfast. When he gets out of the tub, he waddles back into his house to eat. The process is reversed in the evenings; he gets dinner first and then a bath before bedtime. It never ceases to amaze me how good he is and how much he enjoys his routine!
It did take a little while though. Be consistent and try to stick pretty much to the same time of day. Don't expect too much too soon, and after a while your ig will get the idea. Always clean and disinfect your tub after your ig poops; I drain the tub and then refill it with hot water and a little bleach. If it needs further cleaning, I use Comet and then rinse it VERY well with the showerhead. I do this right after he poops so the tub has plenty of time to dry before his next bath. Good luck!