I have always taught my puppies to ask, verbally, simply by not letting them out of the crate in the morning until they DO ask, with a whine or some kind of little noise. I always respond IMMEDIATELY, with a verbal response of my own, like "Good boy! Want OUT? Ready to go OUT? Let's go OUT," emphasizing the key word OUT and repeating it several times all the way out the door. Over time I will respond a little slower, which encourages the dog to make a bigger noise, up to, eventually, a little yelp or a bark. I'm teaching the dog to talk to me, to ask. To communicate verbally. Then when he's reliable in his crate, I leave the crate door open but the bedroom door closed. Now he's learning to keep the room clean as if it is an extension of his crate. And he should still bark to be let out.
When he's older and has learned to keep the whole house clean (one room at a time), he will ask at the door when he wants out. You have to teach him to ask.
