Talking to little cousin about dieting?

today my aunt asked me to talk to my cousin about his eating habits, and loosing weight. He isn't obese, but over the past year and a half his eating habits haven't been the best. He eats a lot of crap, cookiees, loves pasta, dressings, the whole 9 yards. he's gonna be 12 in october, is about 5"2 and anywhere between 125-130 pounds. Like I said, he isn't fat, but I have noticed the weight gain, he's got a little belly, tities, and a chin. My other cousin is an *** and always calls him fat, and it upsets him. My aunt wants me to talk to him but idk how to, because he is a very stubborn and spoiled little boy, both of my cousins are. I mean I am a bit of a health nut myself, having lost over 100 pounds, but my cousin is VERY VERY spoiled, please gimme tips on how to talk to him. He isn't in extreme danger of being obese, but I fear that if he doesn't change his habits, then he will grow up with them, I rather he loose weight now, then when he is in his teens like I did, because it ruins you.