My 15 month old son has just lost a pound?

My son has been low in the growth charts since he was born. He recently had gotten above 20 pound at his last check up at 13 months and now at his 15 month appointment he had lost 1 whole pound to 19 pounds even! The doctor want to set up an appointment with a nutritionist and that's fine, but what i don't understand is that he eats fine and he eats often, he does play a lot and runs around constantly, and doesn't get colds easily. So i was just wondering if anyone can please tell me if my son is ok or if there is anything i can do to help his situation and calm my worries?