I think we can't really make kids to do the "diet thing". Maybe small portions of healthy meals, less bread, less sugar and oily foods. Let him play and be active.☺️
Reduce meal portion and include more healthy food, and increase the frequency of meals
Give him smaller meal portions more frequently and make sure the meals are healthy
Reduce the consumption of unhealthy food and snacks. Let him eat more fruits
Do giVe him smaller portions and less sweets, sugar etc...