Just wanted to lend a supportive ear/shoulder to MaryZ and Shag as I am also the mother of one rather plump teenager and two whippet young adults. These things are never as simplistic as they seem to people who have no experience of it, and you need to tread extremely carefully for fear of giving a fat child a complex and making things worse.
DS3 (13) - very chubby. We struggle to get trousers in his age that do up at the middle. Started off as a very skinny child and then became inexplicably chubby aged nine, and has got fatter and fatter each year he has got older. He hates sport and does very little exercise out of choice, and certain lifestyle restrictions make it hard for him to walk much unless walking for the sheer sake of walking, iyswim. He would always eat junk food and pizza given the option, is a fruit and veg refusnik, but he does not over-eat in terms of quantity as far as I can see. He does do a feast and famine thing though - can go all day without food then has his evening meal and is hungry again in an hour and will come downstairs at midnight and radio the cupboards for crap. I have all but given up buying anything remotely 'treat-like' for this reason, but then DH and the other two moan because there is no food in. 
Luckily the other two do not live with me all the time.
DS2 (17) - very thin, always has been. Very active, likes sport but doesn't really play any these days so he can hardly feel smug on that count, but always full of nervous energy, can't stay still in one place for long. He's always hungry, but a grazer - needs to eat little and often, gets full very easily and will vomit if he gets too full. (not intentionally) He will eat lots of salad and veg, and LOADS of high carb bland food like pasta, rice and bread, and covers everything in mayonnaise or cream, puts three or four sugars in his coffee. A combination that would ordinarily would make someone fat - but not him. Can take or leave meat. He still eats loads of takeaways and junk though.
DS1 (20) - Pretty much leads a sedentary and slug-like existence, hates sport and eats mostly crap. Will eat fruit and veg, to a point, but like most young men his age he has a massive appetite and would always choose meat, chips, fry-ups, takeaways etc. Is not a big fan of sugary things though. He is built like Tarzan; extremely muscular and lean. He likes to walk a lot, but that is it.
How do I explain to DS3 where he is going wrong when he looks at the other two and and sees little or no exercise going on, loads of junk food and takeaways (they buy them, not me) and a very mixed bag of both good and bad eating habits and preferences, and yet they are both thin?
That is a rhetorical question, btw, but just wanted to say it is not as simple as it sounds to change it, and it is not a simple as saying 'he eats crap so he is fat, he eats well so he is thin.'
anyway, back to the mad Algerian......