How to get my little brother to lose weight?

He was diagnosed as overweight about two years ago, but it's getting worse. I have tried to be like, "let's make a June's resolution (like a new year's resolution for all of us people who are really late) to be healthier!" And he agreed but he keeps just eating junk food. I ask him of he wants an apple (or etc) and he says no, he's not hungry, but then I look where we have cookies or other junk food and it's like half gone (we were the only ones home)! My family and I try to encourage exercise (he swims for about an hour a day and will sometimes play outside with his friends) and nutritious food, but he just seems to keep getting bigger! I'm 16 and he's 12 and I think he weighs almost as much as me! My dad doesn't help much, he gives him sweets. What can I do? If I could, I would lose the weight for him, but I'm not him. :( Please help! Oh, and if I ask him how his "June's resolution" is going, he always says it's going well. I just want him to be healthy. :(