Charlie Barks To Come Inside

I'm trying to teach my dog to bark at the door when he is ready to come in. Charlie is already trained to bark on command. He knows "speak" and also my sign-language for when I want him to bark. The transition now is going slow however. Everytime he comes to the door he just whines. He will only bark until I motion for him to do so.

Perhaps he needs a bit more practice. Perhaps I shouldn't call for him to come indoors and just wait for him to ask to come in. He loves being outside and is spending more and more time outdoors.