I just looked back because I remembered your DS has some challenges Cola. Motor apraxia and dysgraphia will impact on him not wanting to write out his work too. If he can solve problems in his head he probably can't see the point of writing down the answers. It's harder then to do so when he needs to when problems become more complicated.
Also, if writing takes a lot of effort then concentrating on that when doing Maths may make it hard for him to focus on the math part. So if he is using all his concentration to form the numbers on the page the meaning of the numbers themselves gets lost.
Does he use grid paper to write his Maths on? It can help organisationally as long as the grid isn't a distractor. I know others have used apps to utilize tablets for doing Maths.
