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Thanks! That's the way I was leaning, I've received the same advice from a few different sources now, and even my son agrees! I totally agree that it's most important to understand everything properly before pushing on. I'm very 'rigid' with that in my 'other life' as a piano teacher... I spend MUCH more time with the 'beginner' stuff than most teachers do, (sometimes years if needed), I don't just automatically zip them through the grade levels like many parents expect, sometimes to their great consternation! But the result of this is that my students are always MUCH stronger than most by the time they reach the intermediate levels, because there are no holes or big weaknesses in their early training.
Before typing that up right now, I hadn't actually made this connection between my STRONG insistence on this philosophy for my piano students, and my struggles with how to approach things with my son. If I step back and ask myself "if this were piano instead of math, and he were a student instead of my son, what would I do?"
And the answer is a big DUH. Level E it is!! 
