When I read your post it was almost as if I had written it!
I, too, have an 8 year old boy who is overweight. He's got a very soft midsection with rolls and I worry constantly. He genetically gets this from both sides of the family. Both my parents and all my sibs are overweight. I'm a very healthy person, but my hubby isn't. Every time I work on a healthy meal plan for my family, they gang up on me. Hubby is a firm believer in veggies, but thinks blue cheese dressing or cheese sauce must be involved. I just try to keep my son active. When the weather permits, we ride bikes together, go for walks around the neighborhood, play tennis, even go to the playground. He is in soccer in the spring and fall and in baseball in the summer. Over the winter, I struggle because I don't want to go outside!
Just try to be aware of what your son is eating. We have a rule about veggies. My son has to eat them first. That's all that goes on his plate. He can't stand it, but if he can't eat anything else, he'll eat them eventually. Then, when he's done with veggies, I give him the rest of his meal. It usually means that he's at the table much longer than we are, but I think it's important. I would definitely eliminate soda. It's about 140 calories a can. My son only needs about 1200-1400 per day. A couple of sodas can ruin that. You might check out a calorie needs calculator online. You can enter your sons age, weight and activity level and find out how many calories he should be eating.
I hope some of this rambling post helps. Good luck. I know how stubborn 8 year olds are.