Get him to do things like 7834095.56 / 143.8 ? Or can he really do that in his head? I'm serious; make the sums hard enough that he sees the benefit of writing it out. It's great that he has strong mental arithmetic too. Sometimes he'll have to grin and bear the requirement to write out working for something he can do confidently in his head, but if for now he needs practice, it's probably better to practise on things he can't do in his head, because that's less frustrating! (At the same time, encourage estimation and other sanity checks.)
In your example 7834095.56/143.8 it doesn't divide evenly (as a terminating decimal). When do you stop calculating, and in what form is the anser to be given?
When I use a calculator I get 54479.10682 (a 10 digit approximation, not exact). But when you use long division, do you know a way of getting that the first digit of the quotient is 5 straight away without some trial and error (and subsequently the other digits the same way). Suppose, when you ask how many 10000's of 143.8 go into 7834095.56, if you carelessly guess 4 of them instead of 5, and subtract those off, you'll realise you haven't subtracted off enough 10000's of 143.8 and you have to subtract off one more before proceeding to the next digit of the quotient. The only way of avoiding this that I can see is to know the multiples of 143.8 up to 9*143.8, but I'd be curious to here other ideas. Can it be made purely mechanical. Of course it would be easier if we all used base 2.
