Something we've found with rebound from stims - not only does their behaviour lash back, but so does their appetite.
But hiding it and hoarding it are not good signs.
For those of you concerned about the poor quality of what he's getting into - never forget, you are the parents and you have ultimate control. Not only can you lock it up, you have ultimate control because you can always refuse to buy the stuff in the first place. If difficult child takes the junk food you buy, then stop buying the junk food. If this means that others in the family have to miss out on the cream cakes and crisps - then so be it. Let EVERYONE eat healthier for a while. It's not difficult to do, and surely is better than the alternative (where everyone has to eat rubbish only).
If you lock up the food and he still gets to it, then be prepared to give up on locking it away. We tried locking the fridge, but difficult child 3 found ways to beat the lock. Putting a lock on a cupboard is no good, if the kid can undo the lock with a screwdriver.
With a difficult child who is hungry but who isn't overweight - we filled the fridge with good food, the sort of stuff we would serve as part of a meal. The kids were permitted to help themselves when they were hungry. So if difficult child came home form school and went straight to the fridge it was OK. The kids could only choose from cooked sausages, cold cooked chicken, raw vegetables, pre-cooked roast vegetables (which the kids would re-heat with the chicken or sausages in the microwave), cooked rice, cooked stews. If the kids pigged out on this for afternoon tea and spoiled their appetite - so what? There's nothing wrong with filling up on what they would have been fed for dinner anyway. And often the kids ate a good meal at a time when they COULD eat it. If I made them wait, sometimes they'd be too tired to eat - after fighting with me all afternoon about being hungry.
When they began to eat only rubbish, we stopped buying it. No more fizzy drink. No more sweetened anything. Only water and milk to drink. For anybody. Yes, we could still have tea and coffee. But no more biscuits in the house, no cakes. The kids could still eat whenever they wanted, but the range got reduced. "Treat" foods had to be earned back - nobody could have any (including parents) until EVERYBODY had earned back the privilege.
If you try to do this any other way - by allowing say, parents, to have treat foods - then the kids will feel that this is very unfair. Again, by only restricting it to one child, then again - the kid is less likely to get the message. It's also leaving temptation too close.
Marg