I am sorry to hear your cat has this tumor. My name is Rebecca. I am a veterinarian and will do my best to help.
Removing a kidney is not an easy surgery. I am a general practitioner, and if this were my own cat, I would not attempt the surgery. I would go to a surgery specialist. It will be a large incision, and he will need to recover for a week or so.
Have any blood tests been done? If his BUN and creatinine are normal, the opposite kidney should still be working. If the BUN and creatinine are high, he is down to less than 25% kidney function, which means the nontumorous kidney is not functioning normally.
This is a hard decision. If the tumor is benign, his opposite kidney is normal, and surgery is successful, he might have many good years left. If there are surgery complications, if the other kidney already has reduced kidney function, if it turns out the tumor is not benign (the ultrasound biopsy is a needle biopsy and may not pick up areas of cancerous cells in the tumor), then he will have gone through an invasive surgery but not have a long life ahead.
Are you working with a veterinary surgeon, and/or oncologist?