Hydee (The Feeding Doctor’s pediatric dietitian consultant) and I came up with some thoughts for this not uncommon problem. Remember though to consult your child’s health care provider to be sure there are no other allergy concerns.
Most of the time in fact fat consumption with dairy allergies is not a problem. Soy milk has roughly the fat equivalent of 2% milk. If your child does not have other allergies, good sources of fat are eggs, nuts (almond butter if peanuts are a risk) avocados (my daughter loves guacamole), meats, cooking oils and fish. Make some muffins with oil, or zucchini or carrot type breads with oil. Many of the prepackaged cereal type bars that many kids enjoy often have fat. Very rarely a child might need a high calorie and fat drink supplement. You should be consulting a nutritionist if there is enough concern to warrant this.
Keep an eye out to be sure your child maintains his growth, but mostly don’t worry about it and be careful not to fall into the trap of pressuring your child to eat because of allergy concerns. (Many moms worry that their allergic kids don’t get enough calories and end up pushing food which often makes matters worse. Kids who are pressured to eat tend to grow less well and gain weight more slowly.)