Son wants to lose weight

My son is overweight. His doctor isn't too worried. He said sinces he's going through puberty and eating more it's normal to gain. But his cousin(live with my sister) have been calling him fat a piggy... He's 14 and now he's extremely self conscious. He started skipping gym so he doesn't have to change I'm the locker room. I found that out yesterday. He is eating less and feels fat. What are healthy ways a 14 year old can loose weight?