If you get angry at the dog (rubbing the nose in it, etc) what you are creating is a dog that will hide to do it's business. The dog doesn't associate WHERE he pottied with your anger, all he knows is that when you see his waste, you get mad - so he'll try to make sure you don't see it. Him putting his head down is not indicating that he knows he was wrong, it is a calming gesture meant to direct your anger someplace other than him. People often mistake these kinds of gestures as proof that the dog knows he is guilty.
If there are no medical issues to be dealt with - then you must start at the beginning for house training. Take the dog outside often, stay with him, and praise like crazy when he goes outside. This is likely going to take a while since it is very possible he will avoid pottying in front of you until you regain his trust. In the meantime - never, ever get mad at the dog for going in the house. If you happen to catch him squatting, then give a firm NO and take him outside immediately. If you don't catch him in the act, don't react. You can also take any poop piles outside to the spot where you take him to potty and put it there. Then when you take him to that spot outside, he'll smell his own waste and it may encourage him to use that area. Clean up all indoor accidents with an enzyme cleaner to take away the smell - if you don't then the smell entices the dog to continue to use that spot.
Livin in a Newfie Drool Zone