My 18 month old is naughty in terms of eating, please let me explain?

He spits out vitamin D milk drinks much less than the 20 oz he is supposed to daily and prefers breast milk, but , prefers juice, eats tofu and puree at least, but does not eat parts of my meals easily. For example, he takes like four to five baby bites of a cheese toast sandwich. He is 2-3 pounds less than other kids his age. Does he take smaller bites because his stomach is smaller or are there any ways I can make him more hungry?