Another on-the-go solution is to buy some maltrodextrin powder from a home brewing shop. It is a fine white powder that also is called corn starch. It is almost tasteless and can be mixed with water in one of the water bottles so that when he takes a swig he is getting a double benefit of rehydration and refuelling. A few heaped tablespoons full in a 750ml bottle is a starting point.

Maltodextrin/corn starch is a the staple ingedient of the energy bars and drink mixes used a lot by cyclists, including randonneurs, and some are mentioned in this thread. The important part is that it comprises long-chain carbohydrates which even out the energy availability without creating the insulin spkes and mood changes that simple sugars will. Plus it's dirt cheap compared with the commercially produced stuff.

I think Mountain Dew in that part of the world also has maltodextrin (as well as caffeine), and you might check Powerade made by Coca-Cola -- here in Australia its primary ingedient is maltodextrin. Avoid Gatorade at all cost as its main constitutent is simple sugar and that's not what you need; same applies to most other common-brand "energy" drinks.

If all that is unpalatable, the bakeries are a cyclist's best friend. Bread and pastries are made of flour (or should be!) and that comprises the same sort of long-chain carobhydrates that are digested and release energy over long periods.

I'd also suggest that you really do need to think about winding back your daily distances now rather than when you get to Mexico. Keeping up that sort of pace for young people can slowly but surely defuse the enthusiasm. He and the others might seem to be OK with the daily distances, but kids have a tendency to hide important feelings from parents.

I also couldn't figure out the attraction of nuts and trail mixes while cycling. They clog up my teeth, and the tiny bits have a habit of being inhaled in to the bronchial tubes when I start getting energetic up a hill. Nuts also tend to take rather too long in my estimation to digest and instead tend to pass straight through. Dry foods require extra water to be taken in, too. Softer, moister foods (back to the bakery again) seem to be a better option.