Being able to explain how you arrive at answers is a useful skill, especially as calculations get longer and more complicated (I'm remembering 2 sides of A4 or more at uni..) going throough teh steps makes it easier to find wher eyou have gone wrong.
TheNumeracy strategy made quitea lot of discussing how you arrive at your answer and part of teh plenary session will often be children explaining how they worked it out and comparing.
I find that some children quite enjoy explaining their methods and it does reinforce that there is not always just one way to arrive at the solution.
However, just as there is often more than way of arriving at a solution, there is often more than teaching method.
Could you explain to theteacher how threatening your son found it, perhaps you could incorporate some explanation into games etc at home.
Simpe things like " countdown"
Take a pack of cards deal 2 to make a 2 digit number (the target number)
then deal 5 more .
The challenge is to make teh target number using teh 5 other numbers, once only, you must use all the numbers, but you can use all 4 operations.
The is the one who gets teh answer first, but teh real value of teh game is taht you each have to show how you got there.. use a white board or piece of paper to jot down as you work. Its not as formal as writing down all the steps in a "sum".
Maths investigations are also good for getting him to figure out how he is arriving at a solution. Have you tried "nrich" which has loads of extension activities for children who excel at maths.
I think off hand that the address is www.nrich.org
, but you can always google it.
The thing that you and his teacher will need to work towards is making the "process" part of teh calculation seem fun, rather than threatening.