RT, without wanting to sound harsh, you seem to be in a bit of a rut in your own attitude to the problem.
He has to learn this, he must learn that. It's very top down and that is total fuel to the fire with an eating disorder.
Lots of people saying 'this is a good idea' but it's not. Youre focusing on his weight and failing to see his feelings.
Ignore the weight. (yes, really)
Ignore the eating. Treat him like you would a sensitive friend, or someone you had to meet with but didn't know well - be ultra sensitive, polite, don't push him into any behaviours. That will follow once he has his own motivation, and to get that, he needs to know he is loved and accepted for himself.
His behaviour has been poor, but if he senses your despair about his weight, he may feel he has nothing to lose.
You can't help being concerned but you sound like you're blaming him. It's only natural to feel this way but please, please stop focusing on his eating because it is a SYMPTOM and even if it were solved temporarily, like with force feeding a person who was too skinny - it wouldn't solve the cause.
It's like having a go at a child for having a fever.
He's miserable. He is, by the sound of it, extremely depressed.
He needs to speak with someone, perhaps, and if you can't get NHS psychotherapy for a whilee, can you afford to pay for it privately?
It would be a lot less than the fat camp thing and also a real long term useful thing for him - if he would be willing to do it.
Mind you sometimes having a parent (who is obsessed with your weight - sorry but it's got to that point by the sound of it) paying for your therapy means it still won't help.
Ask him what he would like to do, if he had literally free rein - would he want to live apart from you? Would he want to stay?
Try and start by saying 'this is nothing to do with your weight, and I don't care if you are fat. I only care about whether you are alright, and I don't know if I can help, but if I can, in any way at all, please let me know'.
And leave it open. Meanwhile take measures to protect your dd's sweets. But forget all the controlling stuff - he doesn't have to stop eating, and he doesn't have to learn to control his appetite. That's his choice. He knows the consequences. The more you push this agenda, the worse he will get.