Chemotherapy has side effects such as loss of appetite, nausea, vomiting, diarrhoea, constipation, blistered mouth or throat tissues (mucositis), dry mouth (xerostomia), changes in taste, and fatigue. These side effects are common since medicines for chemotherapy kill some healthy cells as well while targeting bad cancer cells.
Your uncle may be suffering from one or more side effects listed above. Combination of these side effects makes it very difficult to maintain a good diet for the patient. Soft foods such as soups, mashed potatoes, cereals, etc may be taken when the symptoms are worse. Usually patients cannot tolerate tea and coffee because of caffeine that may cause more dry mouth and nausea. However, you can try if he can tolerate some week tea. Encourage him to drink lots of water, juice and other non-carbonated drinks. He should discuss his symptoms with treating doctors, as they may be able to prescribe medicines to prevent or treat nausea, or blisters in mouth. It is difficult to suggest dos and don'ts unless you are aware of specific symptoms and conditions. It will also be a good idea to consult a dietician who can plan a diet tailored for his personal needs.