My little brother won't eat real food!$?

He won't eat any thing but some little debby cakes chocolate cake and banquet chicken nugget that's the only brand too I asked a previos question but I forgot to mention that he's 11 years old and has been like this for 9 years and as each day goes by theirs another food he won't eat. He won't go anywhere near pediasure and to top it all off he's extremely overweight.... What should we do?!