Vegetarians can get plenty of iron from legumes, whole grains, and dark leafy veggies. However with the further restrictions of the PKU diet it is impossible to get all the iron needed from food. My son's PKU formula supplies some iron and we worked with the dietician to determine how much more he needed to get from a supplement. We add his liquid vitamin supplement directly too his formula for ease.
As a side note, it would be good to look at how much calcium, magnesium, and potassium he is getting in his formula and other sources as those are often low as well in the PKU diet. His dietician can figure out if he is getting enough or not by comparing his intake to reccomended amounts.