How do I talk to my step-dad about my step-brothers horrible diet?

My step-brother is 12 and all he eats is mac and cheese, pizza, and pasta. My step-dad doesn't realize how horrible this is on his system and the lack of nutrients he is getting. I am a nutritionist and personal trainer and he won't even listen to what I have to say, or what my mom has to say. He does not look overweight, but I know his fat content is way above normal. Once he hits puberty and continues to eat like this because his dad doesn't care or understand he is going to be so fat. Last time we went out to dinner he ordered (from cheesecake factory which gives you a huge serving) five cheese ravioli, but instead of getting it with the tomato sauce it comes with, he asked for alfredo sauce instead. He had three sodas with dinner also, and a piece of cheesecake to himself. Its just so insulting to be in the profession I am in and to be completely ignored is just so hard. My step-father is also a big wuss and won't discipline his kid or tell him to eat healthy and lets him do whatever he wants. I'm concerned about his health and I think he is too old to be at a point where he stills eats like a 5 year old. Any suggestions from other professionals or parents?