Sounds like he might be a little insecure with his body weight. If he wants to lose body fat then he is first going to have to accept the fact that he is overweight and then make a mental commitment to change his ways. It's always easier to make that commitment if he has some one alongside him supporting him throughout the way. So if I were you, I would apologize to him and let him know that you didn't mean to hurt him. Also try to help set a good example, even if it means you have to change your own diet to reflect a healthier one. I would also try doing activities other than sitting inside all the time. Try inviting him to the park, throw a football or Frisbee around, or get a group together and go on a hike. Along the way make sure you never poke fun of him, or say anything that could be taken as an insult but always keep encouraging him. A healthy diet and different forms of exercise will be a good start. Hope that helps!