I'm floundering a bit with this one. Any advice? DS (19) believes he's too thin. He's not at all. He's become obsessed with weight gain powders, energy drinks, muscle bulking stuff etc and is spending a lot of money on packs of the stuff but does absolutely no exercise at all. (He used to do a lot.) He's also coming home in the night after going out with mates, visiting MacDonalds on the way home and then making himself enormous pasta meals full of cheese, butter etc, having had a massive main meal already, earlier in the evening. (Last night he polished off half a tray of lasagna from the fridge after a night out, having already had two adult sized portions for his tea plus alcohol .)
In fact he's put on a fair bit of weight over the past year and has a bit of a double chin and visible 'covering'. I wouldn't say he's overweight and currently looks average weight/height but a bit unhealthy. I've tried to broach the subject in general terms, talking about eating healthily, plenty of veg, regular mealtimes, exercise etc but ds gets extremely defensive and aggressive, insisting that he is incredibly skinny and underweight and needs calories. He says 'All my friends think I'm skinny'. He genuinely seems to believe he's really thin and he's just not. 
Is this a weird reverse body dysmorphia -thinking you're too thin when you're clearly not? Is it a response to stress - it has clearly become more of a 'thing' during A levels and starting a stressful uni course? The general advice about eating disorders (if that's what it is) says take the focus away from food and don't discuss weight gain/loss. So should I just be ignoring the stacks of bulking powders and missing lasagna?
Anyone else had this? Any tips on how to deal? All ideas gratefully received.