Have you talked with your son about this? Does he explain it as an accident or having trouble getting to the bathroom in time? Or does he give any other explanation?
Considering the history of cancer, I would definitely check with the doctor. Is it possible it is triggering anything in your son? Is it possible he has a new growth that is putting pressure on his urinary tract or somehow affecting his kidneys?
Our 14-year-old son kicked the Pull-up habit, so to speak, a couple months ago. He has had three accidents at night since then, but I think we might be making progress and we're thrilled. So, I understand your frustration.
Is there anything new in your son's life that is stressful? Has his diet changed recently? Is he forgetting to empty his bladder fully when he pees (day or night)? Where is he on the scale of puberty development?
I have been told that puberty is often a time when teen boys, who still struggle with dryness, turn a corner to being dry. Our son is particularly slow on the puberty development scale. All of his other friends are shooting up in height and their voices are deepening. Our son is just now getting to the stinky feet stage (oh joy!), so this may be progress.
I wish you luck and patience during this time. I hope your son's health is okay. Although, if not, that would help explain this development.
Good luck!