My guess is that he will outgrow it and it isn't harmful to his health
My pups are about five weeks old and aren't nursing from mom anymore but they still try and and they try to nurse from their dad, whose boobies are kinda small I think, but they still try. My guess is that puppies are like human babies and they have that sucking instinct and try to satisfy it. We give human babies a binky or a pacifier. Wish they made some for tiny pups!