I'm really sorry @HollyhockDays - it's so unfair. My mum got breast cancer just before my 18th birthday (recovered and still going strong). Are you and your DH able to tell him together?
Obviously only you know your son best. But, from my experience, I would say to be absolutely open, honest and transparent. Tell him as much and as fully as you can and promise to answer all his questions as honestly as you are able.
My parents were very religious. Fine, but a lot of people around me at the time were convinced of the healing powers of prayer and muttered in vague tones about outcomes and oblique sources of comfort and relief. This didn't help. I wanted to know what was actually happening, how I could help, and what the realistic prospects were. I wanted to be strong in times of trouble. And it was perfectly OK to be told "we don't know yet, but we will tell you when we do" if that is the present case.
Macmillan offer support that you and your son can call on, including a support line. Emotional, financial and physical help for people with cancer | Macmillan Cancer Support. There will also be a student support service at your DS's uni; it might be helpful (if he wants to) to loop them in so that they are aware and can help. Unhappily I had a sh~tstick lecturer at my college who caught me crying on the day I was told and berated me for it, saying "I don't know why you're crying, loads of people get it these days", but she had form and was sh~tty generally to everyone. However, this was 30 years ago now. It was a different time.
All I can say, really, is to be open with your son. It's beyond horrible. But you can face it together. I have no right to align my cancer experience with @mindutopia's, but I completely agree with their experience of a sense of hurt and betrayal in not being told everything and being left hanging in the direst of times.
I sincerely wish you a positive outcome in all things, and happier days to come for you and your family. I hope the consultant appointment goes OK. x 💐