Encourage him to move out as soon as possible after high school graduation. He'll eat a lot less when he's buying his own food.

Maybe when you say "Son, I understand you're growing, which is why I have bought copious amounts of highly-affordable foods A, B, and C. However, when it comes to things like foods D, E, and F I am NOT made of money and I'm pretty sure you sometimes eat way more than you actually need, so I am going to buy a set amount per month and if you eat it all, that's on you and you'll have to wait until next month and stick to what we have left."

Never even after he moves out on his own every time he comes over to visit he will eat you out of house and home. Because it his mother’s cooking.