High school boys are still growing and need lots of protien even if they aren't in sports, but they'll need even more if they are. Be prepared to have something for him to eat when he gets out of school too. Lots of cheese, hamburger meat, chicken--white and dark--with the skin on it, high fat lunchmeats, eggs prepared lots of ways if he'll eat them, pork skins for crunchy snacks, nuts too. My son was in college and fully grown when I put him on the Atkins diet. I didn't try to control his calories at all, only his carbs. He lost 25lbs in less than 6 months. He loved double meat/double cheese bacon cheeseburgers without the bun but with the salad, just make sure the dressings are mustard and/or real mayo but no ketchup. I made taco meat salads with lots of Mexican food additives like cheese, salsa, onion and peppers, but no taco shells. The carb balance tortillas by Mission are very good for meaty burritos and breakfast burritos made with scrambled eggs and sausage, and Sara Lee Delightful 45 cal bread is about the lowest carb that is generally available at stores, but some stores have other versions of low carb breads that would be good for growing boys. Look through the cookbook here for Stef's pizza bake, which is awesome actually. You can put all sorts of toppings on it, especially meaty ones. I kept wieners in the fridge at all times, which also could be wrapped in low carb tortillas with some cheese. Also look for Dreamfields Pasta for making Mac and cheese, or Spaghetti with meat and pasta sauce. Best wishes!
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