My son is a very picky eater...?

He will not eat veggies, fruits, or anything healthy. I've tried feeding him by mouth and he will spit out the food or even throw it up. His pediatrician suggested Pediasure. She says it will help him get all his nutrients, but I want to know from parents if they've tried this or if they have any other suggestions. (My son is 5 yrs old..)