Thank you all for your replies, really helpful comments.
Summerends, you made me smile - when I am getting frustrated with him, I will remind myself that there is a lot of merit in being efficient with your effort! It reminds me, too, that I wouldn't want him to be a perfectionist either, always pushing himself too hard.
Tansie his results seem good, as far as I can tell: end of Year 8 results were English reading 7c, writing 7b; Maths 7b, Science 7C. The rest were between 6a and 6c, apart from Spanish - 5a. His effort levels were 1 for Maths and art, drama and music, 2 for all the rest.
He got level 6 in both Maths and English in his KS2 SATS. I read on here about kids working at level 7 and 8 in year 7, and initially thought that if others were doing it, he ought to be capable of similar. I understood that less than 1% got level 6 in English that year, so assume his potential must match most of the high achievers I hear about. However, I realised that the mixed ability classes at school for years 7 and 8 did not really lend themselves to surging forward at that sort of pace. It didn't bother me too much, as I felt it would only really matter at GCSE level.
In the meantime, we got a tutor for Latin outside school fortnightly. He really enjoys the lessons, does the homework without too much complaint, but it is difficult to get him to do any learning vocabulary, so his progress has slowed down considerably. I am beginning to wonder whether it will be worth carrying on next year, as I don't want extra strain during GCSE years.
Although I did feel that the relaxed pace in years 7 and 8 were fine, now that the pace has stepped up, I am concerned that his attitude hasn't. I really don't want to be nagging him all the time …
It's difficult to know about approaching the school. I don't really know who to speak to. We did speak to the Head of Year 7 at one stage, saying we were worried he wasn't developing a work ethic. He seemed to understand, spoke to DS, and emailed all his teachers but we didn't notice any difference in his effort levels. I suppose they look at his marks and think he's fine, why worry. Perhaps they are right, but my instinct is that where he will come unstuck is when he starts A levels and realises that he has a lot of catching up to do if he is to get the course / uni that wants.
thinkIve - good idea, will give that some thought, and find out what geocaching is!