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Hi, this is Ron. I am a licensed vet with 33 yrs experience. This sounds serious to me, and I think you should bring him in to the vet if you haven't already. I could only guess at the causes for this, and I can think of lots of possibilities. The dark urine and extreme weakness and his age make me suspicious of a bleeding splenic tumor, but lots of other things could cause this.
He should be seen asap.
The splenic tumor is sudden. It grows to a certain size with no symptoms, then ruptures and bleeds into the abdominal cavity causing a profound anemia and weakness. They can become very weak in a matter of hours.
But again, it may be something else. Some liver problems can cause weakness and dark urine.
Retrievers, both labs and goldens are more prone to these tumors, and it occurs in late middle age quite often.