Can I just confirm - who raised this issue that your son is overweight? Did you raise it? Or did he?
This is pretty important in terms of how you approach it. If it's him himself who have said "christ, I'm a stone overweight" then you can be more pro active in your support. But if it's you who have pointed it out, then obviously you want to avoid giving him a complex about it.
I'd say if HE has brought this up, then I would look into getting him a workout programme that he can do at home. Considering his age I would probably recommend either Insanity or P90X (both you can get on Amazon). For p90x you would also need to buy a mat, pull up bar and weights (but you can get these cheaply from sportsworld or whatever). Actually for a teenage boy I would definitely recommend P90X over Insanity.
The good thing is that the programmes have lots of different workouts on them, over a number of dvd's (all in the pack), so you aren't getting bored of the same workout over and over again. They are also incredibly effective (I used to do Insanity, which is "better" for girls as it's just cardio).
The other great points are...
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It comes with a diet plan! So that's a good start for him - as long as he can ignore them pushing their ridiculous juicing products...
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The best thing...if you go onto youtube and type "p90x transformations" you can actually see people who have uploaded pictures of themselves before, during and after the programme. These aren't officially sponsored videos by the company either. It can be a huge inspiration!!!
If it's you who have raised this, then check out bbc food or whatever, and try and find some healthy recipes. He might be whacking salt on everything partly because of his age, so you might want to look into things which naturally have a very strong taste - vegetable curry (SO healthy if you make yourself), home made pizza (not too bad), etc.
Try and revolve meals around vegetables if you can - for example, a vegetarian lasagne (made with canned lentils) has about a third of the fact of lasagne made with beef.
It will be much easier to approach from a "this is a healthy diet" perspective, than it would be a "you need to lose weight" one.