On the toilet!
Saturday night, after having a tantrum when I wouldn't let him leave the table when he wouldn't eat the food he'd specifically asked for, he cried, "I have to pee!"
I replied, "Fine, come to the bathroom and pee. Then go back to the table until you eat the food you asked for."
After he peed he said, "I have to poop. I'll poop on the toilet," coming from a blotchy and tear-streaked face.
Sure enough, he did, with a smile on his face! Needless to say, laughs and congratulations abounded and he was given chocolate, which had been the perennial promise for pooping on the toilet.
Sunday and Monday, however? "Diaper. No toilet."
This evening he said he needed a diaper and asked me to please give him one. "No," I responded, "Poop on the toilet!" He said no, and said he'd put on his diaper himself, and asked me to turn the light on his room. I replied, "I'll turn the light on in the bathroom and put the Sponge Bob seat on the toilet, but I won't turn on the light in your room." After 30 seconds or so he walked in to the bathroom and said, "Okayyy."
Mark ran in to watch. We all cheered. Davey agreed that it was much less messy and he loved the positive attention.
The changing table in his room could be going away soon and a big boy bed may be in his near future.