Update: I spoke with owner of tutoring center and she sat and watched DS regroup for subtraction. I was right, he didn't quite grasp the concept. She went over it with him and explained it to me. I can now regroup! DS seems to get it now and she's going to give him more practice.

Lesson to me - if I think DS doesn't understand something, don't ask him. Check with someone else who knows that method of math. I was impressed with how owner explained the regrouping to him. Unless I know the new method, I'm just going to get DS more confused!