I would start doing the down stay in different places and vary the time you are gone. You have a behavioral and an obedience issue to work through. I would try to get the dog up to about a 10 minute solid down stay, even though only 3 minutes is required for his CGC.
I would move further away and return when he is calm and reward very calmly. Then walk away again and return when he is calm. Vary the time and gradually increase the distance until you can step out of sight. Then repeat with you going out of sight and returning.
Be very careful to not have the person with him praise or reward the dog when he is anxious. This will just encourage the anxiety and tell the dog that being anxious, nervous, etc is ok.
Also, be careful with your praise and reward each time you return. Praise calmly and softly, too much verbal praise or petting will cause the dog to break. If the dog understands the down / stay then a correction is needed for breaking. For this dog the correction may only be a strong negative marker. A firm, calm "quiet" may help when the dog is barking, but this behavior is unacceptable.
I'm not sure this is something that you can fix in a week. If it is only related to his experience when you were away you should be able to. It's is kinda of odd behavior, I would think there are some other underlying insecurities that are beginning to surface. You need to be really clear when dealing with this, very calm and very clear in your communication. The 3 minute down / stay should not be difficult to train and I wouldn't accept this behavior. I would take a bunch of steps back in training and work on a 5 minute down stay with you next to him. Then progress to moving further away. I would back tie the dog and ensure that he could not break. I would let the dog know verbally that the barking, whining is not acceptable. I would not praise, pet or console the dog when it is "acting out," barking or whining. Be very matter of fact and don't give the dog any satisfaction from that behavior.
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