I agree. If he is pooping and playing, and is not skinny, I think I would not worry.
My chihuahua (almost 1 year old) and barely hits 2 pounds, only eats 1/4 of a can of food a day. I leave dry down, he rarely eats this. He is not skinny, he is just built tiny.
And please dont give him nutrical daily. That can actually make him eat LESS. It has lots of nutrients/protein, and can make him alot less likely to eat as much as he might without it.