Hi, Welcome to Healthcare Magic Forum,
You have not mentioned your Father's Age? Since he is Diabetic he has to take his Medications regularly and if he is on Insulin, he has to take regular Insulin
Jabs. Now he has developed Gastroenteritis
, he will need antibiotics and antidiarrheals. Ask him to exercise and go for brisk walk regularly, this will keep his Blood Sugar under control. Give him plenty of fluids.