Underweight and short 9 year old

Does anyone else have this problem? my son just turned 9 and is about 4 ft and 50 pounds. He looks smaller than some 7 year olds. He has severe food aversions and won’t eat any dairy except ice cream and won’t eat protein. Only wants dry and crunchy carbs (will eat plain pasta) and the skin off apples and cucumbers . At my wit’s end and I’m worried about his self esteem since he’s so small.