Chiming in! Agree with most of what's written above - so many factors at play! Glad to hear that you're going to see a swallowing therapist - be sure to ask her if she has experience with H&N Ca.

Might want to try a slight behavioral approach: have the goal be one (tiny) bite of apple sauce every waking hour. A full serving of something - even if it's smooth and easy to swallow - is likely overwhelming on many levels (taste, coordination of swallow, fear of choking, fear of throwing up, fear of never eating again...) Theoretically, if he can adjust to taking in very small bites of apple sauce, spaced out over time - he can be successful - not get overwhelmed- ad build confidence. As confidence builds so too will the amount of bites!
Be patient - he needs to take time - it takes the mouth a very short time to get unaccustommed to food - it is more difficult than I had ever imagined it would be just to get my mouth used to accepting food again.

Good luck!