Oh boy do I know your pain! lol.. my DSS is 16 and bread and mild seem to be his 'staples'....leftover pancakes, toast, you name it. And although he eats a good meal, an hour later he is hungry again. I recall one morning I cooked up a pound of bacon, 10 rolls, a dozen eggs (scrambled w/cheese and tomatoes) a half gallon of juice, and the same with milk. That was for me, DH, DSS(16) and DS(9)....DSS at a majority, and was wanting more food later!! LOL
Point is, I don't know of a cheap snack...all I can think is if you hit the sales at the grocery and get the day old bread, or cheap bread at 50 cents - 80 cents a loaf and put it in the freezer. As I'm sure you know bread takes less than an hour to thaw, so he could get his carb fix in no time.
Like I said, I feel your pain! LOL