C.,
I had a similar experience with my son, showed interest at 16 mos and did both potty and poop on it. Quit after a few weeks. Showed interest sometime before he turned 3 but would only let his Mom help him--which obviously doesn't work when he has a sitter during the day--that lasted 1 week.
Then, 2 months after he turned 3 on New Year's Eve, he decided to start going on potty with an incentive of playing with a NEW highly desired toy for 5 min every time he went. It worked with no accidents.
It took 2 more months to get him trained on pooping but he was afraid. He got a little constipated and got a big sick to his stomach and was forced to sit down and go while I sat with him. After that, he realized he could do it and it took a month or more to enjoy the experience.
Now, he is 4 and the last 6 mos he has spends 20 min each time in there reading books loving his private time.
I didn't know that the early interest in it at 16 mos was typical and then they abandon it. The upside is my son has had virtually no accidents because he is older and has been capable for soooo long, even at nighttime, pullups have been dry almost every time.
It is true, when they are ready they will do it.
A.
A little about me: mom to a charming 4 yr old boy and a bubbly 11 mo old girl