That's a tough one. I actually never had anyone ask me what my prognosis was but several people said things like, "but they caught it early so you'll be fine, right?" I never knew how to respond to that since "they" didn't catch it early and I had no idea if I would be fine or not. Maybe you could just say, "we are very hopeful that he will make a complete recovery," or "we have a great team of doctors working hard to make him well."