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Hello. I'm sorry to hear about your dog.
With a calcium level around 15 and progressive weakness over a couple months, by far the most likely cause is Lymphoma (a type of cancer). This is very common in goldens as well. Most dogs with untreated Lymphoma have a survival time of ~ 2-4 months without treatment. That seems to be where you're at right now. With chemotherapy, they can live ~ 12 months.
The diagnosis can be tough to make if they don't have lymph node involvement and the cancer is internal in the organs. When the lymph nodes are enlarged, we can do a needle aspirate and diagnose this. When it's more internal, you need organ biopsies to diagnose.
It sounds like he may be getting close to the time to euthanize. If you want to treat aggressively, I would see an oncologist to get a diagnosis and then start chemo asap. It's not ideal to wait weeks before treating, but you could still get a partial response and some good quality of life for weeks or months.
Here is some info on Lymphoma for you:
I hope this helps, let me know if you have any other questions.