How do you know it's the acid that's causing the problem? I'm curious how anyone could truly know that. As far as eating alkaline, that's a long-term solution. The folks who are the most concerned about this are those who follow the Macrobiotic diet. You don't have to have any interest in the diet itself to look to it for info on this aspect. What you want long-term is a healthy balance between alkaline and acid, without having too much on either side of the line as it can through off digestion. Now, if there truly was a way to know for sure it's acid causing his problem, and it doesn't actually sound like acid if it's still the problem of throwing up, you'd want to look at what he's eating before exercising. For example, protein requires a high acid environment in the digestive tract so if he's eating, say, protein bars soon before he competes acid production has to go up to digest it. That's why you usually eat them following training, not before. Before, you focus on electrolytes and carbs for energy. Some minerals also require acid to break them down, but others are the opposite. But acid doesn't usually cause throwing up, it causes heartburn and reflux if it causes anything, and in young people, well, they can usually eat pretty much anything and not be bothered by it because of their quick metabolism. Because your son is so active I'm assuming he's metabolizing his food and burning it off pretty quickly and very regularly. You'd notice high acid more if he gets a problem lying on his back soon after eating, or has the pins and needles of heartburn. But if you know for a fact his digestive system is having a problem with acid, then yes, in the long term eating a more balanced diet can help. I'm guessing from what you've said so far that it's not an acid problem but what do I know? Has he by any chance been on antibiotics a lot? Or recently before this started? Have you tried supplementing with a probiotic from the refrigerated section of the best health food store in your area that focuses on bifido-bacteria? Does he eat pre-biotics, such as fermented or cultured foods? All these can help if his problem is a digestive one.
The reason acid comes up is because there is a thought that vomiting during intense exercise is the stomach being flooded with acid to empty it. Like the body goes into panic mode and wants to shut down digestion to focus on other things and then acid comes into play. A lot of athletes take things like Tagamet prior to a race.