I have an average size ds1, tiny ds2, and huge ds3. DS2 is 26 months and 26 lbs, ds3 is 14 months and 27 lbs.
DS2 had milk /soy protein intolerance as an infant but out grew it by a year then fell off the growth chart between 15 and 21 months. We went to a gi specialist who said he is just little. We also saw a pediatric dietician that had him drinking pediasure or boost and gave lots of diet suggestions.
Our pedi said he is probably just little but order the test and gave the gi referral just to be safe.
To get more formula calories you can add more powder to water. We were told 3 scoops of powder for 5oz of water.