Belching is a normal physiologic response to excess air in the gastrointestinal tract. Air is swallowed along with the food or it is produced due to energy dense foods which have a high fat or carbohydrate content. Chewing gum will only add to the problem as it leads to more air ingestion.
I’m a little concerned about his weight which is quite in excess of the norms for children of his age and height. His excess abdominal fat might contribute to his current complaints. I would suggest more play time and diet restriction for him rather than any pharmacological intervention. Coax him to exercise regularly at least six days week. Losing the excess weight will make him fitter and help him get rid of his gastroenterological problems.