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He picks up calculations so easily and fluently, that I could probably teach him the remaining elementary school level calculations in about an hour per week.

The applied work is what he needs more of- puzzles, word problems, analytical problems, etc. He needs to develop strategies to solve these problems such as developing a matrix, lists, etc.

Off topic, but this is exactly like my DD10. She learns anything calculation-related in an instant, and does it flawlessly (it's not just mechanistic--she will easily understand what the process is as well), but word problems involving any type of logic and "how do I arrange this to solve it" are much harder for her. I've not been sure what to make of this as a skillset. I don't know if the issue is carelessness, lack of practice, or something else. She swings a little towards the ASD spectrum and I've wondered if it's related.