As for protein intake, it needs to be per his age and required amounts.
If anyone, takes in TOO much protein, especially in a young child, it can overload their kidneys, for example.
So, dont' just willy nilly load him up with protein.
as for weight gain, healthy 'fats' are also important.... 'fats' and milk fats.... are an important "nutrient" for brain development and bodily growth.
So are you saying, your child is in the "failure to thrive" percentiles?????? You said "he isn't out of the normal range yet on the percentile scale...".... ?
So, is your Pediatrician concerned, or not, right now????
If not, don't worry.
Or, if the Doc is recommending high protein foods, what did the Doc suggest??? They must give you advice/ideas, too.
As you said, your son is a 'wonderful eater...." So, how is it that he is "underweight"????
Or, some parents give their child "Pediasure" which is a "meal" drink for children... and it has protein in it/vitamins/fiber.