It's good that he is able to open his mouth completely. Hba1c is high, consult to diebetologist and dietitian.

Its just starting of radiotherapy, so dont worry as most prople wont see any problems till 3 weeks. He should be given High protein diet, plenty of fluids , I suggest atleast 4 litres a day, No spicy foods, No hard foods, Too Hot or cold foods or fluids to be avoided. If he starts having problems with food intake down the lane, try giving semisolid or liquid food. Contact you diabetic doctor/dietician for personalized diet plan and medications. Donot take Antioxidants in any form  in large amounts during radiotherapy.

Since he has been started on RT to the oral cavity, I would advise you to follow these points : - to give him soft diet like well cooked daliya, khichdi etc - to make the food as nutritious by adding the seasonal vegetables - fruits should be cut into small pieces so that he can chew easily - to avoid too spicy food - you can give him the usual foods which was given prior to radiation, only in a softer form - give him small frequent meals which will boost his appetite - to clean the mouth with saline water/ betadine mouthwash after each meal - to maintain water intake of at least 2 litres/ day