first if you haven't already tried to objectively talk to him about his height for weight match start there. at 7 maybe showing him a growth chart (check on the cdc. gov website) and plotting his height weight bmi <link> and show him where he fits relative to the healthiest zones (for bmi between 5-85%iles is the lowest risk). second maybe there is some teasing going on at school or in the neighborhood that is giving him this inaccurate body image. societal and cultural pressures beyond the schoolyard and neighborhood might be at play as well. media messages about physical appearance are everywhere and are capable of influencing kids this young. we sure don't want the heathy eating and activity message to turn into an unhealthy drive for thinness. lastly could there be something more? maybe. if you can't seem to make any headway on changing your son's concerns about himself then maybe a check up with your doctor can be a way to see if there is something more.
how do i handle my 7 year old son that not only thinks he is fat (only 44 lbs) but also thinks that he is ugly?