So sorry to hear about your son's ordeal. I have a sister who has gone through radiation and chemo or two different cancer diagnoses this year. The first one she drove herself to and from the treatments and then went to work or went home depending on the time of day. She refused all help from me but allowed a 'friend' to assist her. For the second cancer she was in much more pain and lost her appetite. She drank Boost and lots of water and juice. Her doctors were not concerned as this is normal with chemo and radiation. The second time she had a port implanted which was used for the chemo. She was fitted with a bag that had the chemo in it and she wore this bag 24/7 for five days. She would get it removed for the weekend and then on Monday start again. She did this for three weeks on top of 30 plus radiation treatments. Still she refused all help from me.
I know it is hard but you must ease off on your son. He is 54 years old and capable of making his own decisions. I can imagine his throat is extremely sore and will be for a while. Let him know you are there if he needs you but don't nag him. Be thankful that the Pet scan showed no cells. It sounds like he knew you to a T. Let him tell you in his time and his way about his cancer and treatments. He sounds like a determined person. My oldest son is 41, unmarried and lives three hours away. I worry about him but don't let him know it. If he needs me, he will let me know which he has done more than once.
My mother had breast cancer in her thirties and then in her late 60's she got kidney cancer. She refused any and all help with getting to and from treatments and after care. She even went so far as to cancel her plans to be at my house for Thanksgiving that year without telling me why even though she knew she had the cancer. She and I always had a very strained relationship and by canceling at the last minute just compounded my feelings of being the red headed step child.
I wish your son the best. He will recover as his body recovers. I'm sure he will talk with his doctors if he has any concerns.