The sense of smell changes for some people on tx and he may be having an issue with the foods he attempts to eat, they just don't smell the same anymore. Try somethings that are served cold and not hot or warm, like chicken and tuna salads served on crackers or bread, raw veggies not cooked, cold hard boiled eggs not fried, cold pizza, salads and anything else you can think of that's not always cooked. The smell of anything cooking sent me to another room when I did tx, and then I couldn't eat it unless it was served cold and I'm not a picky eater so I think I know how he feels...Loss of appetite is quite common, and on some real bad days we just have to eat to survive, like ice cream, cheese cake almost anything just so we can take the tx meds and get some caloric intake...
Did the doc prescribe an antidepressant, for the moodiness and irritability? It is most likely from the tx and sometimes an anti anxiety med can be used also if he can't take the AD.
He needs to stay hydrated and hopefully is drinking enough water.
Ive been drinking protein shakes with whole milk it will give him vitamins he needs and also some fat grams. If he's losing a lot of weight the make some good shakes for weight gain with more calories. My last treatment I was drinking ensure because I couldnt stand food anymore. Good luck. Joe