A: Your question really represents two problems. The first problems is that your son's sleep is disrupted, which is common in children with ADHD. Plus, medications for ADHD can suppress appetite, so when the medication wears off, your son may suddenly feel hungry for a snack. It is important for his well-being and schoolwork that he gets proper rest. Ask the doctor in charge of treating the ADHD to review the medication type, dosage, and when it should be taken, which may solve your son's problem of waking up at night and wandering around the house.
The second problem is that your son eats sweets at night. I do not know if your feel he is actually "stealing" or if you are just using the term to convey that he is rummaging around the kitchen when he should be asleep. Is your son overweight? If he is not overweight and is otherwise appropriately nourished, eating a few extra sweets may not be so terrible. The main danger from this habit is that he may develop tooth decay from going back to bed without brushing his teeth.
Punishment, scolding, and disapproval are not likely to be in your son's best interest in the long run. Instead, I would try reasoning with your son and suggest that he try eating something like fresh or dried fruits or nuts. Insist that he brush his teeth afterwards (to remove anything stuck to the teeth). You can always cut back on the sweets that are stored in the kitchen and have other types of wholesome food available.