Sounds as though it might work to say "you can start geometry when you like, but you have to understand it'll be important to write careful answers, so you should wait till you're ready to do that". That is, I think just telling him the truth as you see it might work just fine. And he probably will just naturally get better at writing carefully etc, given a little practice and time. I like how many problems with DS solve themselves if I can manage to be a little patient :-)

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