Thanks, very helpful. Have checked out the BMI thing which puts him way up in the 90's at the very top of overweight. 
DH will contact the school re the nurse. As far as special meals go, the LEA offer free school meals to all primary school children. They have a choice of 3 options with a rota system so they all get a turn at going first, but DSS is a vegetarian so only has one choice per day. It's possible that if he doesn't like the veggy option they could be doing him something else (he's not very adventurous about food) and as it is on a 3 week cycle they'd know in advance.
He has said he wants to join the Cubs lots of times, but Mum says she can't take him as she goes to a support group for her MH issues on the same night. DH and I have talked about him volunteering to take him, but it would have to be a swap for his midweek contact (when he takes him to football) so not really a net gain. I haven't got any experience of cubs - would it be the kind of thing that would build his confidence?
I took both boys for a long bike ride this weekend and DSS mentioned that his bike at his mum's is broken so DH also offered to mend it (has been 'broken' for a year) when he dropped him off tonight but he said the response was hostile, which probably means there's nothing wrong with it but it is a good way of coping with not wanting DSS to be riding on it (dangerous).
But wise words from nunnyfickname we've also been talking about our contribution to the problem. DH admits that he tends to buy chocolate after football and give DSS fizzy drinks. When we go out sometimes the vegetarian menu can be limited (and DSS is very reluctant to eat out anyway) so he tends to be allowed to have just chips, which I would never let DS get away with.
I need to get more info about veggie diets - I usually just shove a meat substitute into whatever we are having, but if, say, we are having a homemade pie, I'll get a readymeal type substitute for DSS. I never give my kids ready meals so it's laziness really, but i'm also not very confident about being able to give him a balanced diet so prepared stuff seems safer, IYSWIM.
Phew. lots to think about.