Thank you Cecilcaterpillar and octsmum for replying.
Picking one word a day and trying to use all of its synonyms throughout the day seems like a very practical idea.

Regarding maths...he has learnt all of the topics but I've been spending some time going over his previous work and a few common topics are cropping up as being the ones he is regularly making mistakes on- so I will be working on those for the next couple of weeks.
And I've never got him to mark himself...I do question how much of my feedback has any real effect so this is an interesting alternative which I will try.