Gayatri, good to hear from you after a long time and that too with good news. Finally, your dad quit smoking and is on way to recovery. Post-radiation healing takes long time and depends from patient to patient; for some it may take anywhere to an year to become normal and pain-free.

You need to pay special attention to his diet as Christine had mentioned, increase protein intake. Go for Dals (Pulses/legumes). Egg whites are also good options. Discuss with doctor for some Protein supplements. Ask your father to drink as much of water as he can, it may be difficult as water also tastes different after radiotherapy. Try coconut water, it has cooling effect - you may have to filter it using tea strainer.