What you'll find out is that once kids start eating with other kids you have no way to be certain that your kid will eat all those healty foods you pack for them or if they ditich it and eat the sugary cereal that their friend brought along. As for juiceboxes - you can get many juiceboxes that are 100% juice - you just have to pay attention to the labels. Bottles of water are fine too although the camps usually have giant water coolers available all through the day for the kids.
Think crackers and cheese, dried fruit, if peanuts arent allowed see if seeds are - you can get dried fruit & seed bars from Trader Joes or Kashi brand at the grocery store. Yes - they include a lot of sugar - but if he can't bring a ziploc bag of nuts it's tough getting them protein during the day any other way. Chips like tortilla, bagel or pita are a decent option too.
While I know you say you don't buy many pre-packaged items realize that if you buy all these small plastic containers to re-use there's a very good chance they won't make it back home as kids lose track of everything at camp. So you might end up saving money, your time and effort by buying a few things that are prepackaged like salsa, hummus, cheese, etc.
Fresh fruit like apples (whole) and oranges (quartered in a ziploc bag) are some of the best options as they're refreshing - a drink & snack in one.