I have not personally tried it but i have a friend who is a nurse who gave it to her son. Her son has never been an eater. You could also try the toddler formula. Enfamil is called Next Step but Parent's Choice makes one too I think. It would have more nutrients. You could also ask his doctor about a multivitamin for added nutrients.
I give carnation instant breakfast to my 21 month old son and he gained like 2lbs in like 3 months. And I gave it to him because a nurse form WIC told me to, she said they wanted him to gain some weight..he is also a picky eater. Oh and when I put it in his milk he tells me ice cream..lol I give it to him every morning with his breakfast.