>>I watch him as much as possible, and he rarely goes inside. But, inevitably, I cannot be there 24/7 for him (I work 40 hrs/week, and sleep 6hrs/night)
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>>But, my friend's dogs always either whine or scratch when they have to go. Is that something that Yeller will automatically pick up when he gets older?
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As he gets older, it is likely that you and he will develop a schedule and communication that works for you. But that only happens if you are really alert to him now, and get him outside ample times so that he doesn't start the habit of relieving indoors. When you are at home and awake, that's when you need to take him out at least once per hour. His body has to catch up for the waste that accumulates during his sleep and crate time, etc.
When you cannot watch him, he needs to be in a safe place such as a crate or small room, so that pottying around the house does not become a habit. He's not actually learning anything in there, so it's important to keep him in the room with you under your eyeballs for as much of the time as you possibly can. Supervision 100% of the time means he's either in the room with you watching him or he is in a safe place such as his crate or a small room--100% of the time.
Sometimes people are busy and think they will put off housetraining until later, and in the meantime just clean up after the dog. That's a huge mistake, because the result is the dog forming the wrong habits. The easiest time to housetrain is right from the start. But as far as the dog signaling you, that's a side thing, and in my opinion not a good goal. It puts far too much responsibility on the dog.
An accident in the house is always the fault of the human, never the fault of the dog. There's an old joke that a rolled up newspaper is a good housetraining tool. Every time the dog has an accident in the house, you take that newspaper and hit YOURSELF with it, saying "Bad, bad owner! I should have been supervising my dog!"
There are details on how to housetrain a dog in the Housetraining articles at the link below my signature at the bottom of this post--free, of course.
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Kathy Diamond Davis, author, "Therapy Dogs: Training Your Dog to Reach Others," 2nd edition, and the free Canine Behavior Series articles at http://www.veterinarypartner.com/Content.plx?P=SRC&S=1&SourceID=47