PET
If the spot on the lung is getting larger it is likely cancer. The best scan for this is not MRI or CT but PET imaging. Is he getting a PET scan? Ask about a PET scan. (CT/MRI can only tell if a spot is there but not if it is cancer. PET scans can tell with a fairly high degree of certainty if something is cancer or not.)
thoughts
Generally, once a cancer has spread, removing the primary tumor does not help because the cancer is already elsewhere. Chemotherapy, immunotherapy, radiotherapy, and palliative surgery are the remaining options.
Renal cell carcinoma studies disagree, but there is some evidence that even in cases where the cancer has already spread, removing the primary tumor seems to improve life expectancy. This is still being debated.
The urologist wants to remove the remaining kidney because there is cancer present in it, and if the lung spot is not cancer, perhaps removing the remaining kidney will completely cure your husband.
The oncologist feels the cancer has already spread from the left kidney to the right kidney and perhaps to the lung as well. He feels that removing the remaining kidney will not cure him. He has seen time and time again where there are other "spots" that are too small to be seen on scans today, but will pop up over the next few months in the liver, lung, and even brain. He does not want your husband to be on dialysis. He does not want your husband to endure another big surgery since it will likely not be curative. When cancer has spread beyond the primary site, the only way to attack all the places it has spread to would be via the bloodstream, such as chemotherapy or immunotherapy.
I do not know who is right today. But if nothing is done in the immediate future, the oncologist will be right by default because this cancer will spread if it has not already.
I would get the PET scan and see if the spot on the lung lights up. If it does, then the oncologist is the one you want to listen to. For what it is worth (which is very little) my gut feeling is that the oncologist is right. I am often wrong, have been wrong before, will be wrong again in the future, so don't base your husband's care on anything I have said, it is just my two cents since you said "any advice most appreciated."
Lastly, both the kidney and the lung lesions can generally be treated with radioablative or cryoablative procedures. The tumors are frozen or fried. This is not really major surgery, it is done by a radiologist who slips a tube through the skin and places the tip of the tube in the middle of the cancer and either incinerates it with heat or destroys it with cold.
In your husband's case, this may be a good option to pursue. It is certainly worth getting a second opinion from an interventional radiologist that does the procedures. You can ask your oncologist for referral to one. In the links below there is also an animation of Radiofrequency Ablation as well. It is far less invasive than removing the remaining kidney and perhaps your husband will not wind up on dialysis.
Radiofrequency Ablation Links
Lung:
http://tinyurl.com/ylwtwl Kidney:
http://tinyurl.com/yfc2cu