He won't put down the fork.?

I am a heathy person, so are my kids. We eat healthy and excise everyday. My Husband had a kid with his first wife; who I raised since he was 3 now 7. Last year he went to satay the whole year with his mother. When he came back, he was a little bit chubby. Gained over 20 pounds. Now I can't get him to join the family. All he does is eat. His mom ships boxes of junk food here for him. He steal the boxes and hides them i his room. Can you please help me out and give me some ideas.