J.L.
Washington University School of Medicine has a study for overweight children where they set them up with a dietician, give them lifestyle advice, monitor their weight loss, etc. I don't know what the age restrictions are for it or when the available appointment times are (probably not on the weekends). I've only seen the flyer in the pediatricians office. This may not be what you were looking for.
I would also suggest speaking with your sons pediatrician. He is probably already aware of a weight issue and will have access to, or information on, more programs that you could implement to help your child learn more healthy lifestyle choices. Good luck!