Until you get the behaviourist sorted - don't take food away from him, don't take anything away from him (unless its a life or death matter!), without realising it, you (or his previous homes) have taught him to guard "stuff".
See it this way - I love chocolate (true!). I go to stay at a friend's house (someone I don't know very well), and every day they put a lovely big plate of Green and Blacks in front of me. After letting me eat a bit, on day one they take it away from me. Hmm I am a bit surprised but oh well. Day two, more chocolate - oh good, they are letting me have some more. But yet again, they take it away. I begin to consider a) eating it really fast or b) I know I'll try and hide it when they come to take it off me. They come, force me away from it and take my chocolate...... After about a week, I'm onto them, I know what they are up to. They are gonna give me the stuff I like most in the world, then they are gonna take it off me. Well, I'll show em. Next time they do that I'm gonna give them a good slap, that'll stop them. And - it did
I slapped them, and they didn't take my chocolate. (Are you still with me ?
). So, tomorrow, I'll slap them again, cos it worked today.... and if they still try, perhaps I'll slap them harder the next time.....
OK I know I'm being silly but I'm trying to explain it to you in terms that are easily understandable from a human point of view.
Now if every day I was at this friend's house, and they just gave me my chocolate and let me eat it, without interfering, I'd be so much happier and relaxed, I might even share it with them. And - what about if they gave me a bit more when I'd finished it - I'd think WOW I really like this person.....