I get you, OP.
Some people are just hogs and will take whatever they find as their god-given right to eat.
However, you can manage it, if you're not prepared to talk to him - put the baby's peanut butter in a hard-to-open tupperware container, or similar, so he can't see it, then he'll get his own.
Put your snacks in similar containers so he can't see them, then he'll get his own.
Tell him that if he runs out of snacks, he's to buy his own because you've already bought sufficient for normal appetites.
My DH has a chocolate habit - he keeps telling me not to buy him more chocolate, but if I don't, he''ll just raid the kids' stash, which is not on, so I buy him his own. Then he eats it and complains that I shouldn't have bought it for him.
I'm not his mother, and I don't have to facilitate his willpower, but I DO have a responsibility to protect the children's food from his ravages.
I also hide my own snacks/chocolate so he can't find them, because it's the only way to keep them.
So yeah... I have empathy because I have one of these hogs myself, although he doesn't over eat in general terms.