Yesterday's drama with the borrowing and carrying ended thusly: We went to the computer. We fired up the drill pages on the MathUSee web page and suddenly he could do them. Easily. Instantly. I still don't understand what was going on, but it seems to be over. Today he woke up and made his own worksheet. You can see part of it here, although he did it in yellow marker and I took this picture with my phone:
Then he opened up some more online drill practice and pressed on until he had 20 correct. I didn't tell him to, but he just did it. So, he seems to be working it out on his own.