It really is the most frustrating time. You are right; you go from the high of being done with tx and then the garbage starts. It hopefully will be slowly getting better in a few weeks. He is in the hardest part of the battle right now. Do what you can to push him without ticking him off too much!!
We finished tx the 3rd week in October and Kev ate some of Thanksgiving dinner that year. Everyone proceeds at a different pace though, but know there are better times coming.
It would be good for him to begin taking things by mouth. There are meds that can numb things up for a few minutes so he can swallow.
Blessings,
Kathy