buy him a personal gym equipment for himself. desidnate a particular room for it. make him proud and motivated to achieve rather than making him feel like a nebbach.
take him out for qaulity father and son time. Make him feel special and worthy. take him bowling, rollerblading, iceskating, bike riding… there are numerous fun outlets that will actively make him fit. buy him a good branded pair of runners, t-shirt and shorts for his work out. he won’t want to disapoint you after all that effort. in the end he will feel better abuout it himself and will be eager to do it on his own without you encouraging it. at this particular stage they are going through a large growth spirt and it is likely if the food habbits are not funneled in the right way it can cause a major impact on the weight. encourage healthy eating for the entire family, not just him. remember you all need it!