I had a partial nephrectomy on 09/16/11 to remove a tumor 14mm that was solid. I received results last week that it was stage 1 clear cell kidney cancer that did not spread to lymph or other organs. A CT scan was performed 4 years ago and the tumor was a 8mm "simple cyst" per radiologist. so it grew slowly over 4 years. I am thankful however that it had not spread outside of the kidney as it would mean more treatment and less chance of survival. I see your post is old and so this is probably not helpful to you but hoping it may be for someone else.
I personally wouldn't allow it to be pushed back. How old is your son?
My daughter needs to surgeries to correct her heart and esophagus, and the thought that were going to wait at least 5 more months is a horrible wait. I would get a second opinion and push for the surgery to be now rather than later. (Unless there are strong valid points to wait) Cancer in itself is often better treated sooner than later as well! The longer cancer has time to spread the harder it will be to get rid of. On the kidney side; the kidneys are fragile as is and waiting only prolongs the negative affects.