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 :-)
Yes you're right, and he is old enough now for me to explain just what I'm thinking, and I try to. I had told him that to do the Geometry he would need to write solutions in complete sentences and his response was "Complete sentences?! Wow!". Anyway, the more immediate problem is that he needs to write out his Algebra solutions, so a future need to write out Geometry solutions can serve as motivation to write out Algebra solutions now.
