I feel for you... I have had to take a step back and watch what I say so as not to cause trouble.
My DSS is 11 and overweight. We got a kinect for exercise, a bike, go days out that involve exercise (swimming, laser tag etc) but he doesn't seem to lose weight.
I spoke to DP about what he feeds him as I take nothing to do with it (issues with DP ex as she was causing problems between us and mixing things to cause trouble).
I explained he needs to stop feeding him pizza, fry ups, hot dogs and start giving him proper meals. I started making dinners for the 3 of us as we get him every weekend but he would just pick at it or I would be met with "DSS won't eat that" so I gave up as I refused to keep wasting food.
I don't discipline DSS as i don't feel its my place and DP doesn't back me up anyway (I booked us all a holiday abroad this year which was DSS's first and when I would tell him to come get more sunscream on I was flat out told no. DP didn't back me up and so DSS ended up with a burnt face, shoulders and head and moaned about it the full week.
That was the last straw for me, the refusal to listen to me and of DP to back me up ended up ruining my holiday as we spent a week hearing about how sore he was and were in the room every night after dinner so he could watch tv then sleep as he was "sore".
I resolved then to take nothing to do with feeding him, clothing him (bought him a load of new clothes and he either stained them by not listening when I said to lean forward when he ate or by leaving them at his mums (affectionately known as the bermuda triangle) and only spend time with him and go days out etc.
all parenting is left to DP (i do occasionally remind him that DSS needs a shower etc as the hygeine is lacking slightly).
Honestly, I'd love to be able to discipline him and actually feel like a parent rather than by stander (with DP support obv) but DP doesn't follow through with his claims to back me up so I'm happy to leave him to it... working well so far.