Advanced Youth Sports Nutrition

I have three sons, all of whom play football. This week they begin two-a-day practices. They usually don't eat before practice because they will throw up while working out. But during two-a-days, they practice for two and half hours, break for one hour, then practice for two and a half hours more. They are starving between practices but can't eat heavy before the 2nd practice. Any ideas on what to feed them that won't make them sick?

My son is 15 and will be trying out for the football team this summer. He is strong but never seems to have the right amount of energy to finish practices. He played lacrosse this spring and he was always hungry and tired. Any thoughts on what I should do? Also, his coach wants him to put on 15 pounds but this seems like it would make him too heavy for his frame. Or, would it be muscle weight>