Just make sure he isn't on any "low fat" or "low carb" foods - yogurt should be full fat yogurt, same for cheese.
Try giving him things which are high in carbs - chick peas, bananas, rice (brown is good for him), whole grain breads, eggs, custards and puddings made with whole milk (not skimmed or 2%). miami fraxel
From your measurements both his height and weight are just below the 5th percentile for his age. He was born premature so he will falls below 'normal' children growth chart. As long as he growing and gaining weight at a steady rate, no matter how slow, he is developing normal. Suggest in place of milk (milk, fruit, and ice cream blended) or yogurt smoothies (yogurt, milk, and frozen fruit blended). Add dry milk to oatmeal, mashed potatoes, gravies, creamed vegetables, and creamed soups (add additional meats to soups). This will give him additional calories and protein for weight gain. Use all regular dairy products and no low fat foods so he gets the calories he needs. Hoped this helped you.