My toddler is refusing to eat solids?

My 14 month old baby boy is starting to eat solids less and less every day. When he turned 12 months, he was only 18 lbs and the doctor put him on Pediasure. He loves his Pediasure, and now he will refuse anything besides that. I don't know what to do as I don't want him drinking only milk all day. I know he needs to be eating more solids. He will refuse to eat anything I give him that is not sweet (like chocolate) or his cheese puffs. He will not eat any veggies, meat, or even fruit if I give it to him. All he wants is his pediasure, puffs, or something sweet. I need him to eat more food. What did you do parents who were in this situation?