I would agree with nyslp. Don't make a problem where there isn't one. It's better for your son to be chubby now than to have body image problems for his whole life.

I would stop buying soft drinks and have the whole family drink water most of the time. Let soft drinks be something you have once in a while. 

 When it comes to exercise, I'd look at his schedule. If he only has gym class once or twice a week, then you ought to see if you can get him to register for some active class, like martial arts, basketball, whatever strikes his interest.

The focus, though, should be health and not a little bit of puppy fat.