Maybe you can start out a day by showing him an empty cupboard and saying, "Look, Sweetie, no biscuits today!" with an exaggerated

face. "But if you DO get hungry, we can have some of..... THESE!!!"

and hold up apples or oranges or something else. "I know you like the biscuits, but we don't have any more right now, and I don't want you to get TOO hungry, so this will be right here on the counter/table/wherever when you get hungry." Very matter-of-fact. And then move on and see how busy you can keep him.

Also, if you're out of the house, like in the yard/garden or out for a walk, does he ask then? Perhaps if he's away from the house when he'd otherwise be snacking, you can use that to change the habit more painlessly, or bring along a snack of your choosing or mutual choosing between 2 alternatives you pick, like apples or raisins.
Good point about the possible thirst, too, especially now with the weather warming up; I know my kids are a lot more active and more hungry/thirsty all around.