Maybe a visit to the doctor to make sure nothing else is going on may be a good idea, it seems there is an emphasis regarding food and eating .How old is your son ? If he is going through your personal things he is upset about something have you had a talk with him , how is school going any problems there ?
I have tons of fresh fruit and nuts, dried fruits etc available to all of my three children to have as much of as they please. I have also tried giving him more to school and at home. But it seems he will always go and take something that is for instance set aside for dinner or since I do catering he will go and take stuff that is meant for other people. On weekends they are allowed to have other snacks like sweets and chips, during the week however they can have any of the healthy snacks as much as they like. They also have three good meals a day, plus they get yoghurt and a fruit inbetween. It gets better at times, but lately it has only gotten worse. He even goes through my handbag in my cupboard while I'm in the bath, as well as my drawers.I am convinced this has nothing to do with food as it doesn't matter how much I give him, he still does it.
How old is your son, it seems to me he is unhappy about something, what do you mean when you say he steals food, if you mean at home from the cupboard and fridge, why is that stealing, if he is hungry or if we are hungry we go and eat ,if it is at school I suggest you pack some extra food as it could well be he is hungry, growing boys need good healthy ,filling ,food .Have you asked him why he is taking the food at school and what has he said .Has he any siblings ? ..You say there is always some bad behavior you have to punish for, may I ask what the behavior would be ?