My nephew is allergic to milk. With the concern of soy not being good for children what can he drink?

Options. There are several alternatives, although some do not taste great and others have low nutritional value. Options include almond, coconut, oat, rice, and hemp milks. Make sure you check the labels and read the nutrition info.
Soy issue overblown. Except for some premi infants, i see no reason to avoid soy in infants or childhood. After weaning, a kid doesn't need a milk product.Kids can get all their calcium from non bovine cheese, or Tums (calcium carbonate) antacid tablets & drink water. The rest of the diet can make up the difference. Many consider a kid milk allergic when s/he is just lactose intolerant. If true, many milk products would be ok.