Hi Chazygirl~
My 12 year old son is 5' 6 1/2" and weighs 135 lbs. He wears a size 12 men's shoe! It is so hard to judge a child on weight/height alone. I saw a huge difference in my son's size since he started going through puberty. He just got taller but began eating us out of house and home. (that is a topic that I need to post!) His weight gain may not only have to do with going to his "other home" but the stage that he is going through. I agree that he needs to be active and get exercise. Perhaps help him join some sport leagues (football, baseball, soccer or etc) or extracurricular activities (gymnastics, swimming, karate). If he was doing something that he enjoyed, he wouldn't even realize that he is exercising!

Best of Luck!