I am sorry you are going through this. We had a lot of Obsessive Compulsive Disorder (OCD) problems with Wiz. Possessiveness about food was one, esp after he came home from the psychiatric hospital. The psychiatric hospital had mostly unlimited food as long as the kids ate it and they were not majorly overdoing it, which Wiz did for a while.
When he came home we had to decide to either give in and let him have his food to himself, with no one allowed to touch it or to let him have just a few items that were just his and the rest were shared no debate allowed.
We chose to let him pick two items of food that were HIS. One could be a sweet or treat and we only purchased one serving per day. If he ate them all in one day then he was out for the week. He did try to blame his sibs for eating his food, guests, even husband and I, but we knew it was not the case. Regardless he did NOT get more unless we saw the other person eating the food AND knew that difficult child did not offer it. If he offered to share he had to check with us first or no replacements. (This was tried to get us to keep more of the item in the house so he could go through our room and break into locked containers to search for more.)
The other food item was one he could have anytime, much as he wanted. It was something that the rest of the family did not have so it generally was a fruit or veggie that most of us did not eat. It was "his" and his alone. If it wasn't what he really wanted, well, that was OK with us. He still had it to fill his "mine mine mine" need.
ALL other foods had to be shared. No debates/discussions were allowed. If he started he got sent to another room or was given a task to do. Period. I didn't want every conversation to revolve around this and he would have if allowed to. Sometimes I had to bluntly say "You are not allowed to discuss this. Change topics, be silent, or leave."
If we bought something with 10 items in it, say a package of indiv pizzas, he would decide that 2 of them, at least, were HIS. Often he thought more were "his", but this was based on him deciding that one or more of us really did not want/need them - NOT on reality. I would purposely give him less, or give another person more than what he decided was "fair". It sounds mean, but was designed to help him get out of the way of thinking that every item that came in had 1/5 that was his. because if he didn't get his portion he would have a huge rage or tantrum.
I just didn't want him to get to the point that everything we brought in had to have some set aside for him. Life doesn't work that way.
The whole thing wasn't fun, but it did work. Time and patience and eventually he learned that he is not entitled to a set amount of everything that is purchased. Unless HE purchases it or it is specifically purchased for him to have.
The hardest part was refusing to discuss it. I would tell him he was arguing, or justifying (telling me why it should be his way), or whatever. He was not allowed to justify or debate if we made a decision. Not at all easy to be consistent over, but it DID give us more peace than we had in years after about 2 months of getting the hang of it.
Maybe this will give you some ideas to help with this. I am sorry that Obsessive Compulsive Disorder (OCD) issues are a problem. SSRI medications can help if he can take those. They do with Wiz.
