Son is underweight.. help


My 2 yo will be 3 in June so 3 months. He weighs 24 lbs. He is not wven on the growth chart and the ped keeps saying to add calories to his diet like butter and fats . I also have pediasure and ive been at it for 2 months and no progress. They do not seem concerned, he is super skinny , he does eat but in moderation. He wont finish his plate , hes always snacking. I took a look at him and his knees are kind of concerning me , WHO DO I SEE FOR THIS THAT HE IS UNDERWEIGHT?