About 20 minutes later, while at the breakfast table, Mel says "I have a burp." That normally means he has to throw up but we brought him to the potty anyway and he peed again!
Then before his nap I say him down for 30 minutes, nothing. I asked if he really had to pee and he said, "Yes, hug me." So, like all the other times, I did. I expected it was the first of many hugs and then crying and "I'm scared" but this time he was quiet for a minute and then exclaimed, "I did it!" And lo, he did. No drama. Just pee.
We've clearly turned the corner. It took four days.