If I was the one buying all the groceries, and then is also paying for the more expensive protein snacks, I would be pissed too. Did you calmly explain to him that you prefer for him not to eat your "special" snacks. I think if you approach it in the right way, he would understand. If not, then he better kick in some money. LOL
Hubby, and I always used to eat off each others plates, and it never bothered me. Now since my WLS we eat separate meals. He doesn't like most of the stuff I eat, so it's not a problem.