My son 11years old everything he eats, he can just start eatting and have gas, why? No pain just gasie.

Depends. Flatulence (gas) is caused by undigested or partially digested foods traveling to the large intestine, where they are fermented (digested and metabolized) by the bacteria that colonize our colon, often producing gas in the process. The most common gas producing foods include: cauliflower, brussels sprouts, dried beans, broccoli, cabbage and bran.