DS (18) still eats me out of house and home just as he did from about age 13 on. He is about 12.5 stone, 5'9ish and looks just a bit better covered than a stick figure. I think his weight is mostly muscle and bones, and he has big feet. He puts away about five eggs a day, a mound of pancakes or two bowls of cereal, fruit juice, sausages for breakfast, and eats about half of what I cook for dinner for five of us when he's home from college. His friends do likewise. The parents in the house where they all hang out together buy huge quantities of snack foods and fizzy drinks. I buy them a bottle of wine or two at Christmas. I would still prefer to see DS emptying family packs of doritos and downing Coke to drinking lager in some park somewhere.
Why do you see wanting to eat what his friends eat as rebelliousness? It sounds really harmless to me. If this is the worst thing he and his friends want to do, consider yourself lucky.
If you're concerned about his weight, consider that he may be about to do some serious growing, or that he might benefit from participating in some sport or doing weightlifting. DS played sport all through school and burned it all up.
Or maybe you could serve larger portions to him at dinnertime, or allow him to help himself, since the meals you serve are healthy and nutritious?
Is your DS obese? Is he just a little bit pudgy? Somewhere in between? If a medical professional hasn't voiced any concern then I don't know if you need to be as worried as you seem to be about the biscuits.
The way you put eating biscuits and rebelliousness in the same category has me wondering a bit I have to say. Knowing that what he can't have is somewhere in the house is going to backfire on you all, imo. You run the risk of getting genuine teenage rebellion mixed up with the topic of healthy eating, and putting off the development of a healthier approach to food in your DS as a result. The teenage years are to some extent about allowing your DC more autonomy and leaving some decisions up to them; hiding the biscuits seems to be going in the opposite direction.
Would you consider giving him a weekly allowance that he could spend on snacks for himself and when the money is gone then sorry, no more empty calories, or trips to the cinema, or whatever else he would want the money for?
You would have to lead by example here too, and not keep stuff in the house that he craves.