Tell him to go out and play. He's 11. Just go be a normal kid and stop playing video games all day.
When I was 11 I was eating cheese burgers, drinking sodas all day, cheetos, etc.
I also also skate boarding, riding my bike, jumping out of trees, practicing martial arts, and competed on the swim team. Insanely active. I was burning at least 2000-3000 cals a day, for sure. That's what kids used to do. All day and all night.
Clean up his diet, sure. More importantly, go do something except for play video games. New generation of kids are mostly indoors or on the computer.
Not knocking your nephew. I have family that's just like that. The biggest problem isn't diet, in my opinion. It's lack of activity.