His name is Harry Melling. He's actually lost a lot of weight, and he's starting to work in theater. In fact, he lost so much weight prior to the filming of "Deathly Hallows" that he had to wear a fat suit and face prosthetics to portray the overweight Dudley Dursley character.
He says that he lost a lot weight because he didn't want to keep being typecast. He had to move around a lot during drama school, so he got more exercise, and he started eating more sensibly. He dropped from 224 lbs to about 160. More on Harry Melling's weight loss here.