I found that during treatment I lost my appetite completely. I had mastered a lot of foods prior to the treatments effects kicking in. I think the opening to the esophogus shrinks after awhile. I had the same problem where mentally I would want to eat more solid food but it just wouldn't go down the hatch wihout getting stuck. IT was very painful and it would make me throwup too.
Its so frustrating. I tried every day to just eat small bite of soft solid fruits and this helped. At first it burned like hell and it felt like it was gettinig stuck but I think it must have slowly opened it up more and more. It kind of felt like heartburn or indegestion at first. Honey dew melon and catelope felt nice because it was nice and cool and not too acidic. Pineapple too. I think Pineapple has some healing properties as well.

Keep trying. He could also have his esophogus stretched byt getting a procedure done. Many people here have had that done and it seems to help with swalloing. I'm so sorry. It must be so frustrating!